We were staying in a hotel recently. 5* but dated. There was a pool bar near the pool with plastic chairs where you could order some lunch.
We sat at a table right by the pool and ordered lunch. He didn't put a t-shirt on because he didn't want to get suncream on his shirt. People regularly ordered drinks at the bar in dripping swimming outfits only.
Manager came out to politely remind us of the requirement for clothing to be worn at lunch. We had assumed this only applied to the shit down restaurant that offers a more formal lunch. Would you expect to need to wear a t shirt at a pool bar?