The tendancy now is for ships not to heat their pools to save fuel, whether this will change will the advent of cheaper fuel is anybody's guess. I would suggest that you stick your toe in before making a final decision. It would have been better if you had mentioned which ship youhad in mind but the answer might well have been the same. ......Neil.
You can't tell what the weather's going to be like in the Baltic. It can be very cool, but when we went in mid-June a couple of years ago it was hot and sunny for almost the whole cruise, up to 30C. It was certainly warm enough in all the pools.