I think many who know how to swim may underestimate the power of water at times, including me. The water on the river Khwae for example flows very quickly in places, yet people love to jump off their partyboats. I was also swimming off the phraya nakhon cave beach a year or so ago and it was very difficult to swim back to my start point due to the direction of the waves. In the end I swam to the other side, away from the side if the beach I wanted to go and that allowed me to get closer to the shore and walk back. Not a big deal, but I was tired after that. If I weren't a decent swimmer or a person who doesn't panic, I'd have been in trouble there.