One of the great treats of going to Brazil is a trip by boat to swim in the river. We find a nice spot with a sandy river beach, ignore the resident caiman, and pile in to the caramel coloured water. The water isn't cold, exactly, but it feels cool after the roasting heat of the sun. I love the tropics, but don't care for the heat very much, so I value any opportunity to get cool. The current is surprisingly strong, so you can use the river like a natural, water-treadmill. You thrash away as hard as you can, then find that you've made barely any progress relative to the bank, but when you've swum a little way, you can let yourself drift back, to start again.
The visibility under the water is very low, which is slightly unnerving. We know for sure that there are a lot of piranhas in the water, and every now and again you get a little, gentle nibble on a limb or your back, which keeps you on your toes. The reputation of piranhas is much more ferocious than they are (for a start, many species are not carnivorous), and it's quite safe as long as you don't have any open wounds. Similarly, the caiman just keep out of your way.
I know that it's not an efficient stroke, but I think that breaststroke is wonderful for wild swimming. You can see where you are going and hear all the bird calls bouncing off the water. As you look ahead, your sweeping hands slice the calm, virgin water cleanly, making you feel as if you're the first person ever to swim in the water. Even if you're swimming with others, you can point yourself towards a bit of open water and it's just you, your hands and the river. Bliss.