The most common and most likely answer to this crossword clue is the 5 letter word BATHS
I've seen this crossword clue in Financial Times
B
A
T
H
S
Buddy explains!

The straight part
We know that "somewhere to swim" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case somewhere to swim = baths.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does BATHS mean?
BATHS noun- A tub or pool which is used for bathing: bathtub.
- A building or area where bathing occurs.
- The act of bathing.
- A substance or preparation in which something is immersed.
- To wash a person or animal in a bath
- A former Hebrew unit of liquid volume (about 23 L or 6 gallons).
- A room containing a shower and/or bathtub, and (typically but not necessarily) a toilet.
- A lavatory: a room containing a toilet and (typically but not necessarily) a bathtub.