Nowadays, "Dominance" is a word that gets thrown around easily. For me, I dont feel the need to be the "ALPHA" when doing this simple things like bathing a dog. All you need to do is put yourself in the dog's point of view and try to find out what makes him scared of it.
Lets put your example " DOG DOESN'T WANT TO SWIM IN THE SEA", can you think of anything that scares him?....................
O.K. here it is, most dogs are afraid of the sea (at first) because the "FIRST TIME THEY STEP ON THE WATER, THEIR FEET MAGICALLY DISAPPEARS! It just FREAKS them out. That's the reason why GUNDOG RETRIEVER TRAINERS introduce their puppy to water by letting them FETCH a ball so that the puppy doesn't have to think of the SCARY STUFF and let his PREYDRIVE take control of his emotions to a point a pup doesn't realize his already swimming.
The biggest mistake you'll ever do is forcing him like carrying or pulling him to the water.
Here's my video of introducing my Labrador pup to the water:
Cebu Dog Trainer
I used my pup's social attachment to me by getting in the water first and calling him.