In this case sauna can be a good option, as the heat can help reduce the accumulated humidity (not only from being in the water but also the once caused by a wet swimming suit worn for longer time) and in the 20s, you are likely to have plenty of Qi!
In my 20s I used to teach swimming and was freezing all the time. I used the sauna, not to sweat, but to warm up to normal. I think using the sauna this way is fine, especially in winter when it's cold out.
In this case sauna can be a good option, as the heat can help reduce the accumulated humidity (not only from being in the water but also the once caused by a wet swimming suit worn for longer time) and in the 20s, you are likely to have plenty of Qi!
In my 20s I used to teach swimming and was freezing all the time. I used the sauna, not to sweat, but to warm up to normal. I think using the sauna this way is fine, especially in winter when it's cold out.