Wiktionary
swim like a fish
vb. (context simile English) To be a very strong swimmer.
Usage examples of "swim like a fish".
Ryan knew that his son was able to swim like a fish, but the thought of sending him down into those menacing, unknown deeps was simply impossible.
He had heard Lester brag that his daughter could swim like a fish.
It came toward them at the speed of the river, but appeared to swim like a fish.