As a former swimming instructor I can attest to this lol. The best way to stay afloat is to move your legs and arms in slow circles. Many people who drown actually kind of freeze their legs and just flap their arms around.
Also, adults can learn to swim (enough to stay afloat at least) in a single class. I was shocked by how much easier it is to teach an adult to swim than a kid. They just have better control of their bodies and can follow instructions better.