I remember once I was swimming across a lake with a friend in high school and part way through I felt just so exhausted. Like, my arms and legs just didn't want

I remember once I was swimming across a lake with a friend in high school and part way through I felt just so exhausted. Like, my arms and legs just didn't want to work. It's a bit different of a feeling than when you're tired while running or wrestling (the two sports I played in HS) -- it's like you just cannot move. Even in those sports I never felt as dead as I did trying to swim across the lake. It probably wasn't as scary as your experience, but it was absolutely terrifying, especially since -- like you said -- above water you seem fine, but under you're frantically swimming trying to stay above water.