With both small and medium, you can just strafe back and forth in front of your opponent if you're faster than them. With medium and large, you can just circle them and absorb more punishment than them when you're larger than them.

Multi-player would solve the AI problem -- especially if it was more than 1v1 -- and then it would be more challenging.

But I would still request realistic wind and sailing dynamics. That strafing maneuver isn't really possible with real wind dynamics, because you slow down as you swing around and that allows the opponent to catch up to you and return fire. You could have a little arrow in a corner of the screen that shows the wind direction. You don't even have to make the sails movable, you could just have your speed be proportional to how orthongal your sails are to the wind direction in the simplest implementation.

It would add a whole new element to it that would make it much more challenging! :)

By contrast I found winning with the medium boat required the most skill, and lost the first few times I tried.