First suggestion of mine: Craftable anchor points/mooring posts for dock making.
Reason: Not all islands are aesthetically pleasing to have ships parked off the shores, some have great natural bays but others aren't forgiving with ship anchoring, especially along some of the coasts. This suggestion is to add points that would allow a ship to go into it's anchored state, even when it isn't in shallow waters. The objects would not be allowed on ships however due to the function, but would allow proper looking docks to be made in places that aren't natural harbors. If a ship is to enter the anchored state next to one of these objects, dynamic ropes could be shown that would show the ship is moored to the dock or a man-made surface under the water from where the sail posts are. This would allow ships to enter the anchored state further away from the shore, as long as it's next to one of these placed objects.
Second suggestion of mine: Ocean Currents
Reason: It would add a bit more realism to the game with ocean currents. Riding one could either hamper your speed or greatly increase it depending which way you're facing. The currents if you were to pass over one could even affect the trajectory of your ship if you don't correct the pushing changes. These would be placed on the map like how the ship paths are for npc merchants, but the segments could be adjusted on their strength. Rarely with how the current is placed, if the bends are very narrow where it's close to another segment, or the turn is very tight, a whirlpool could appear spinning in the direction the current is turning towards which would cause increased strain for any ships caught within it, the strain growing greater the closer to the center the ship is (Faster decay to describe it better). This could happen when the wind is going the opposite direction of the whirlpool during these bends and if it is strong enough.