I have a Labrador Retre - uh, I mean, Yorkshire Terrier.
Jackson has always been a water boy. But I also always have encouraged him to try new things and be confident in what he does. Always associated it with positive things. Even with things such as jumping up on a log, or on my back, etc, I really like him to be as confident as he can be.
He was not introduced to water until about... oh, 7 months old, I think? I got him in December and pool opened in May. He did not begin diving in until his 2nd summer where I bought him a lifejacket.
I've just always made it a fun thing

I think it's a natural thing for him, for the most part... but I do think one can be trained to enjoy it. I dogsat for a boy who just recently was introduced to an indoor swimming pool and hated it the first visit and is now beginning to see how fun it can be!