[Butz]

I think it would be really cool to have a harpoon cannon on the front of ships (maybe a new ship type).
Fire the harpoon at another ship, and if it hits, then it gets stuck in the side.
If the ship that was hit tries to flee, it shouldn't be able to move until over 50% power, and should pull the attacker along with it.
If the ship is moving, the harpoon's hold is weakened, and it will eventually fall out.
Crew members should also be able to walk across the tow rope, allowing for boarding parties in the deep sea.

I think it would be pretty interesting, since one ship could harpoon an enemy in place, and a second could fire upon the now-disabled ship.

You could make it so you'd have to craft harpoons for them. Normal harpoons for a certain amount of iron, and perhaps gem-tipped harpoons that cause damage to the ship as well.