The game has been touted as "Whatever you can see, you can get to", but that cannot be. Sooner or later you'll have to either come up against some kind of barrier to prevent you from indefinitely traversing in 1 direction, be you on land/sea.
Land is par for the course. Environments, subterranean lairs and craggy peaks. But water? How unfathomably cool it would be if you had some type of device (sextant maybe) that Geralt could use and then as he finds clues and treasure maps, can navigate to those places by ship and explore the briny deep.
It will also be cool if there are multiple types of ships, from small skooners to larger frigates; and I would think they would be very hard to come by. Using skiffs in deep water should be a very harrowing experience, especially if weather makes the seas become fowl.
Either way, I want there to be a level of immersion that makes me feel connected to the world around me. Exploration can be treacherous. Some places can be very unforgiving, and that is the inherent danger of an adventure.