I voted multipe as well.
Damage Model: I would like to see the rest of the damage ideas implemented. However, it's pretty good overall right now and getting better with each update.
Open World: Everyone has touched on this a lot so not much more to add. I would like to see as an immersive environment as possible to explore and have encounters. Not just instanced combat scenarios. I think a huge part of this game will be the graphics and overall beauty of the game.
Add all ships and customization: I'm not sure how many ships are completed now but I always like to see more ships. However, quality of ships > quantity of ships. If there is 5-10 ready then this could possibly wait a while as that may be enough to start with.
I would like to see detailed customization at least some time in the future. The ability make your ship your own will add some depth and help avoid a sea of doppelgangers.
Overall, the sailing is good in my limited experience. I've only tried the Victory (which is great) so i'm excited to try some other ships.