Am new to the forum, here to suggest an idea that likely has been suggested many times, but that I couldn't seem to find in a superficial forum search. Here we go:
A big barrel, 3/4 a sloop in length, 1 middle gallion deck in diameter. It has a steering wheel in the back, along with a periscope which, similarly to the spyglass, has a glare. It has a hatch that can be opened while submerged at the cost of the vessel quickly filling with water. There are bulls eye windows, but the maximum viewing distance is very limited, similar to diving regularly. For weaponry, there is a torpedo bay in the front which is basically a cannon but the projectiles go in a straight line and are considerably slower. It also allows for players to disembark from the vessel through the bay at the speed of sword lunging into water. Sail controls are replaced with dive controls which adjust the depth of the vessel. If it is submerged for too long, it will start to burst and leak. Naturally, the whole thing is rather cramped as is usual for a submarine. The only thing I'm not sure about is propulsion - whether it would have a motor or some kind of fin that uses underwater streams (basically wind but the direction is unrelated) - and size (of both the ship itself and its crew). I think it would introduce some interesting tactics, especially some that require patience to set up the perfect trap rather than simply driving up to an enemy and firing. Do let me know what you think.