Sterling makes pretty awesome ropes. Just bought a new Fusion Nano from them!
i think you'll be happy with either for a workhorse rope. I'd recommend the dry treated Sport out of the two. I don't have experience with either, but I agree that you should have a dry treated rope especially if you're going to be dipping it into the ocean. Don't forget to wash it on occasion!
Maybe other people can offer first hand experience with the Marathon series of ropes?