Open-Water Swimming in the Greek Islands: Guide

The eight island open-water entries, as they appear in the catalog

This list is pulled directly from the bib2bib catalog (discipline "OPEN WATER", located on an island), not typed by hand. Dates reflect the most recently confirmed edition of each event as of the source check on 28–29 September 2026.

• GO Kea Open Water — Saturday, 25 April 2026 · 1.5 / 3km · Kea, Cyclades · official site goevents.gr/go-kea
• Alonissos Challenge, Golden & Silver Swim + Junior Swim Challenge — Saturday, 5 September 2026 · 1 mile / ½ mile / 150m · Chrysi Milia, Alonissos, Sporades · official site alonissoschallenge.com
• Serifos Sunset Race, swimming races — Sunday, 13 September 2026 · 750m / 1,500m / 3,000m (plus kids’ 100m / 300m) · Avlomonas Beach, Serifos, Cyclades · official site serifosrace.com
• Spetses mini Marathon, swimming races — Saturday–Sunday, 3–4 October 2026 · 1.5km and 3km · Spetses, Saronic Gulf · official site spetsesmarathon.com
• Santorini Experience, Open Water — Saturday, 17 October 2026 · 2.4km · Volcano to Old Port of Fira, Santorini, Cyclades · official site santorini-experience.com
• Swim Grand Prix Mykonos — 29–30 May 2027 · 10 / 5 / 2km (plus teens/kids) · Mykonos, Cyclades · official site swimgp.com
• UltraSwim 33.3 Milos ("Aphrodite" edition, #18 of the series) — 3–6 June 2027 · 33.3km over 4 days and 6 swims · Pollonia, Milos, Cyclades · official site ultraswim333.com
• UltraSwim 33.3 Skopelos ("Mamma Mia" edition, #20 of the series) — 17–20 September 2027 · 33.3km Classic or roughly 17km Build, over 4 days · Skopelos, Sporades · official site ultraswim333.com

UltraSwim 33.3: the international series drops anchor on two Greek islands

UltraSwim 33.3 is an international series of multi-day, point-to-point open-water events that began in Montenegro in October 2023. Its first Greek edition ran on Skopelos in 2024, with Skopelos returning every year since without a gap — 2025, 2026 (edition #15 of the series, 18–21 September) and 2027 (the "Mamma Mia" edition, #20 of the series, 17–20 September) — while Milos joins as an entirely new venue for edition #18, 3–6 June 2027.

The 33.3km distance is covered over 4 days across multiple swim segments — on Skopelos the route includes a channel crossing to neighbouring Alonissos, while the official Milos page describes passages through caves and narrow channels. A shortened "33.3/2 Build" option of roughly 17km also exists for swimmers who do not want the full distance.

The organiser itself gives a sea temperature of 20–21°C in early June on Milos and 23–26°C in mid-September on Skopelos — the "warmest venue" in the whole series, whose overall temperature range across all its locations is stated as 20–27°C.

• Wetsuit: permitted but not required; wearing one even once keeps a swimmer in the wetsuit ranking category for the rest of the event.
• Escort: every swimmer is always within 50m of a kayak, backed by a kayak-support boat, a Race Director, a RIB and a dedicated medical-support boat.
• Age: a minimum of 14 on the day of the event; swimmers aged 14–17 need an adult guardian who is also a registered participant.

Alonissos Challenge: 19 years at Chrysi Milia

The 19th Alonissos Challenge falls on Saturday, 5 September 2026, with the Golden Swim (1 mile, two 926m laps) and Silver Swim (½ mile, one lap) starting at 17:10 from Chrysi Milia beach, and the Junior Swim Challenge (150m) starting at 16:45. The organiser itself describes the race as "an institution" for the island, in its 19th year.

A personal swim buoy is mandatory and swimmers must bring their own; a wetsuit is allowed but disqualifies the swimmer from the official ranking, and any flotation aid (kickboard, fins, hand paddles and the like) results in automatic disqualification.

Spetses mini Marathon: Greece’s largest open-water swimming programme

The Spetses mini Marathon began in 2011; swimming races and children’s events were added in 2012. By 2013 the swim already had over 600 entrants, a number Wikipedia describes as the largest for an open-water event in Europe at the time.

In the 2026 edition, the 1.5km race swims on Saturday, 3 October (start and finish at Poseidonion Beach, 60-minute cutoff) and the 3km race on Sunday, 4 October (start at Kosta Beach, finish at Poseidonion Beach, 120-minute cutoff). Wetsuits are explicitly banned in the races, while the 3km requires a mandatory personal swim buoy; the course is marked with buoys every 200m and escorted by RIB boats and the Coast Guard.

The official page states no explicit minimum-age rule; the age-category tables imply a different floor per distance — the 1.5km categories start at 12–14, while the 3km categories start at 18–29 with no younger bracket, i.e. an effective minimum age of 18.

The rest of the Cycladic summer fixture list: Serifos, Mykonos, Santorini, Kea

The Serifos Sunset Race runs Sunday, 13 September 2026 at Avlomonas Beach, with swimming events over 750m, 1,500m and 3,000m plus kids’ 100m and 300m races (the day before, Saturday 12 September, is a separate 10km/5km running race at Vagia Beach — a different sport at a different beach). Its own official-site photo archive goes back to at least 2015, with editions most years since (2020 appears only as a scaled-down "Training Event", and 2022 has no entry at all). The official page states no wetsuit or escort rule — this guide does not fill that gap with a guess.

Swim Grand Prix Mykonos, part of an international circuit that also runs in Portugal, is scheduled for 29–30 May 2027 on Mykonos, with 10km, 5km, 2km, a 400m teen race and a 200m kids’ race. The official page does not state a founding year or how many previous Mykonos editions have run, so this guide does not claim one.

Santorini Experience (17 October 2026) includes a 1.5-mile (2.4km) swim from the small harbour at the volcano to a purpose-built finish arch at the Old Port of Fira, with experienced lifeguards and floating support craft along the route; the official page asks swimmers to arrive at the port at least an hour before the start.

GO Kea Open Water (25 April 2026, Gyaliskari beach) is the only event in this guide where a wetsuit is explicitly mandatory rather than merely allowed; a personal swim buoy is also mandatory "for every race and route", with disqualification for swimming without one. The official page does not mention a previous edition, so this guide does not present it as "established" the way it does the others.

Sea temperature by island and month

The figures below are multi-year monthly averages from Seatemperature.org/Seatemperature.info for each island, for the month in which this guide’s race there is held. They are climate averages, not a forecast for any single year.

• Kea, April (GO Kea): average 16.3°C (range 14.4–18.9°C) — still on the cool side for comfortable swimming.
• Mykonos, May (Swim Grand Prix): average 18.5°C, rising to 21.9°C by June.
• Milos, early June (UltraSwim): the full-month average is 22.6°C; the organiser itself gives 20–21°C specifically for the first days of June.
• Alonissos, September (Alonissos Challenge): average 24.7°C.
• Serifos, September (Serifos Sunset Race): average 24.4°C (range 21.8–26.7°C).
• Skopelos, mid-September (UltraSwim): the full-month average is 24.4°C; the organiser gives 23–26°C specifically for race days, with an August monthly average peak of 26.2°C — the warmest figure among this guide’s islands.
• Spetses, early October (Spetses mini Marathon): September averages 25.2°C, falling to 22.4°C in October (October range 20.3–24.5°C).
• Santorini, October (Santorini Experience): average 21.8°C.

The safety rules that actually apply: wetsuit, tow buoy, escort

There is no single "safety rule" for open-water swimming on Greek islands — each event sets its own, inside the broader World Aquatics framework. The world federation sets a 16°C floor as the minimum water temperature for an official race and makes wetsuits mandatory below 18°C — thresholds that, according to research the federation itself cites, leave a safety margin above the temperature at which some swimmers can no longer maintain their core body temperature.

On the islands in this guide, the sea is almost always above the wetsuit threshold during race season — so most events do not mandate a wetsuit for temperature reasons, but regulate it instead for fair-competition reasons: excluded from the ranking at Alonissos and Spetses, explicitly mandatory at Kea, optional (with a category switch) at UltraSwim.

The tow buoy is the more consistent element: mandatory at Alonissos, at Kea, and in the Spetses 3km. Boat or kayak escort is described explicitly only on the official pages that mention it (Spetses: RIB boats and the Coast Guard; UltraSwim: kayaks within 50m, a RIB, a medical boat; Santorini: lifeguards and floating craft) — where the official page says nothing (Serifos, Mykonos), this guide does not fill the gap.

Which race to pick first

For a first taste of island open-water swimming, the shorter distances with clear cutoffs (750m on Serifos, 1.5km at Kea or Spetses) are more approachable than a multi-day 33.3km event. Bib2Bib’s separate guide, "Open-water swimming races in Greece in 2026", covers further events off the islands (such as GO Milina–Alatas and the XTERRA O.W.S. Challenge in Voula) and is updated on its own schedule, apart from this one.

Three Greek open-water swimmers already have their own bib2bib profile, sourced to the official World Aquatics and Hellenic Swimming Federation records: Spyridon Gianniotis (Olympic silver in the 10km, Rio 2016), Kalliopi Araouzou (two World Aquatics medals at world championships — Barcelona 2013, team; Kazan 2015, 5km — no Olympic medal, 11th place at Rio 2016) and Spyros Chrysikopoulos (the reported Othonoi–Santa Maria di Leuca crossing, 91.4km). None of the three profiles is tied directly to a race in this guide, but together they show the level of Greek involvement in the sport.

What this guide does not include

• It is not a complete national open-water calendar — only island events with an accessible primary source. GO Methana (a peninsula, not an island) and the Skyros Sports Festival Swim (a "discovery-lead" source, not yet confirmed) are left out for the same reason.
• It gives no training, nutrition or medical advice for open-water swimming.
• It does not publish any athlete’s bib number.
• It uses no personal athlete details beyond public competition facts already published by official sources.
• Prices, start times and entry terms change — each event’s own official page always takes priority over what is written here.