When you're doing a water swim, you're really just… well, swimming.

Note that 水泳 will usually refer to swimming as a sport or organized activity. When talking about swimming as a general action, 泳ぎ and 泳ぐ are more commonly used.

This is a jukugo word that uses the on'yomi readings of the kanji. You've learned this reading for 水, but not for 泳, so here's a mnemonic to help you:

The only way you can swim is by chasing after acorns (えい). If somebody isn't dangling an acorn on a stick right in front of your face, you'd never get the motivation to swim. Good thing you're this into acorns.