Tropical waters: 3mm shorty or full. Warm-temperate waters in winter: 5mm + hooded vest. Cold-water destinations: 7mm or drysuit.
Wetsuit thickness depends on water temperature, dive duration, your personal cold tolerance, and how many dives a day you're doing. The same person who's comfortable in a 3mm in 28°C water on the first dive may be shivering in the same 3mm on the fourth dive of the day.
Tropical destinations (28–30°C water)
Maldives, Indonesia north (Bali, Komodo north), Philippines, Caribbean, Bahamas, Honduras, Belize, parts of the Egyptian Red Sea in summer. A 3mm full wetsuit is the universal answer. A 3mm shorty works for single-dive holidays but on a 4-dive-a-day liveaboard, the long sleeves and legs prevent end-of-day chill. Some divers use a 1mm or 2mm 'fast suit' for the warmest sites and the 3mm full for everything else.
Warm-temperate destinations (24–27°C water)
Egyptian Red Sea winter (Brothers, Daedalus), Indonesia Banda Sea, Philippines Tubbataha, Maldives southern atolls in winter, parts of the Caribbean in winter. A 5mm full wetsuit is ideal. Add a hooded vest (1–3mm) for the deeper dives where you'll spend longer at depth.
Cool-temperate destinations (20–24°C water)
Galápagos central islands, Socorro winter, Sea of Cortez, parts of Mexico (Pacific side), South Australian wreck sites in summer. 5mm full + hooded vest at minimum, many divers prefer 7mm with hood, gloves, and full booties. You're cold by dive 3 if you're underdressed.
Cold-water destinations (15–20°C water)
Galápagos Wolf and Darwin, California, Northern Australia winter, South Africa, parts of New Zealand. 7mm semi-dry full suit with attached hood and 5–7mm gloves. Many serious divers in this range use a drysuit with a thermal undergarment for full-week comfort.
Cold-water destinations (under 15°C)
Antarctic, Sub-Antarctic, North Atlantic, Russian Far East, deep cold-water wreck diving. Drysuit only. Trilaminate or compressed neoprene drysuit with heavy-weight thermal undergarment, dry gloves, hood, and dry boots.
Pro tip: bring a 1mm or 3mm hooded vest as a layering piece even on tropical trips. It weighs almost nothing in your luggage but doubles your warmth on the deeper dives. Cost: $50–$100, fits anyone, lasts years.
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