Take the dog to a lake, river or pond and check him out....explore the shallows together...no pushing or prodding. Bring a favorite fetch toy and have him retrieve it in the shallowest ankle deep water....slowly experiment with getting the dog to go deeper....observe the behavior...any reluctance, go back shallower...maybe not even make the dog go to the "deep end" the first outing but keep it fun. Once the dog gets the feel for deep enough water to actually swim, it should be a nonevent. Many people on this forum have GSDs which love to swim, mine included but a proper indoctrination is important as you don't want to screw it up....and then again...your pooch might go charging into the water and love it. Just observe the dog's behavior and go with it...encouragement without prodding.
Oh, a bath versus going swimming are two different events to a dog...I believe...