@liilliil Great question. It depends on people.
I like to have at least 22-23C, which means that June-October tends to work for me unless it's unusually cold.
Folks who are OK with 20C get an extra month on either side of that.
Of course, with a wetsuit, it can be year-round, the water still averages 15C at the coldest of the year and is never below 13C.