Isn't the small boat Geralt controls too small for open seas?
Maybe there could also be smugglers around the coasts of the maps also.
To my knowledge the boat he will use is fine for open seas, or at least you can get fairly far out in it (You'll be using it to travel across any body of water in Skellige, much of which is off the Islands and open water), the major problem would be a storm, now that would fuck him up. Obviously it's slightly silly that he can travel from Novigrad to Skellige just in his little dinghy, but that still makes far more sense than opening your map, selecting a Skellige fast travel point and left-clicking.
I'm sure there will be caves and various other enemies hanging around the coast ready for a fight.