When the PC is able to swim well? If the player trained athleticks or has a specialization in swimming?

Yeah, Strength -> Athletics -> Swim.

A naturally strong person would still be a better (or maybe just as good as?) swimmer than a weedy trained and specialised swimmer, unless you decide that you need the specialism (or training) in order to be able to swim at all.

The ability to swim would be part of the character background. Some can and some don't know how. Do most dwarfs know how to swim? If a character just uses strength for his swimming test he could probably dogpaddle around slowly. Someone trained in athletics could swim better and faster. Someone specialized in swimming could swim like Michael Phelps!!