You’ll have more confidence, and be better able to explore, if you have footwear (e.g. old trainers, jelly beans etc) and goggles. A normal surfing wetsuit, a sleeveless wetsuit top or a specialist triathlon wetsuit will all help you stay warm longer. Make sure you bring towels, a picnic rug, midge repellent, suntan lotion, sunhats and plastic bags for all your wet kit. Inflatables are popular but make sure people won’t drift away on them, especially non-swimmers. A proper buoyancy aid (about £40) is safer, and fun too. 

If you come across a magical pool on a walk it’s quite possible to swim even without any kit. Wear your undies or go naked if it is secluded. If you have no towel wipe most of the water off with your hands then sacrifice one item of clothing to dry yourself with or travel with a small, light cotton sarong.