Bather
/ˈbeɪðər/
Definition 1 of 2
Pro Tip 1/3Water Leisure
Use 'bather' for someone enjoying water activities, without implying they're doing serious swimming. 
The bathers at the resort lounged in the pool, chatting and sipping drinks.
Pro Tip 2/3Not Professional
'Bather' suggests casual activity, not professional or competitive swimming. 
The children, bathers at heart, splashed each other in the shallow end.
Pro Tip 3/3General Location
Use 'bather' for someone in any body of water, like pools, lakes, seas, or rivers. 
The lake was full of laughter from the playful bathers.