What wetsuit thickness do I need for 70°F (21°C) water?

Most divers are comfortable in a 5 mm at 21 °C; warm-blooded divers get away with a 3 mm plus a hooded vest, and cold-sensitive divers on repetitive days will want the 5 mm with a hood. Charts are guidance, not physiology — if you chilled on dive two last time, size the suit to that memory. Fit matters more than millimetres: a flushing 7 mm is colder than a sealed 5 mm.