My boy swims in Lake Michigan. Not sure about salt water but I'm sure its not that bad for dogs, and I would just recommend hosing the dog off after each swim.
They can swim pretty well, and they get better the more you do it. I don't let him go that far, usually he just swims in to fetch a ball and then comes back out. I've been on a smaller lake and also a pool, and my boy actually floats without kicking his legs so he doesn't get tired if he's just in one spot. They can swim pretty fast too, I'm a fairly good swimmer (used to be a lifeguard) and my boy easily kept up with me when I was swimming freestyle.