Bodies of water, such as the lake in question and the sea, are referred to as bathing waters. If you're ever on an English beach and you want to know if that particular stretch is clean enough to go in the water, you check the bathing water profile for where you are. https://environment.data.gov.uk/bwq/
In this context, bathing is used to mean swimming. For example, a public pool is referred to as the swimming baths. It's just British English.
Important question: does it have garlic bread tho