Naho it is !
Most of Naho's geography (from what I remember and believe about it) is eroded by water, creating hazardous, rocky areas. Everything else seems like it would be reefs.
Swimming seems like the most obvious, but it also seems too simple. Besides, if they can manipulate water, all they would have to do is to jet forward (as you suggested in point 2). However, I think we could combine all four concepts into a single sport (nix the sail ship).
What about a race where Nahoins jet across the surface of the water (riding surfboards), while also navigating the hazardous waters (above-surface reefs, rocks, stronger currents, etc.)? In addition to trying to get 1st place, racers are graded on stunts and performance, receiving penalties if they don't do well.