The sea is massive and impressive, but underneath... eh, a wee bit barren. Not really much to find under water, besides the occasional mermaid statue near islands and hungry sharks.
So, here's an idea; take notes from the diving bell.
More specifically, my thought is a little round ball with exits in the top and bottom that could be used to dive incredibly deep in the ocean to salvage wrecks and the inky depths, with a connecting handle up top for players to use like the capstan to pull back up when signaled. My figuring was you could get voyages from factions to salvage undersea area's, get a location, then use that location to dive in that spot to find what you need. then, you find treasure and items there, load them in the bell, then get pulled back up to the surface and go flog the loot.
This could also be an excuse to introduce new types of chests and items, or at least new fish types that you could try and grab the same way you get chicken 'n pork.
Also i imagine doing it while your fighting a kraken and finding yourself in just a pitch black void with orange kraken eyes staring back at you that you could shoot to deal extra damage.
How exactly you'd keep it on the ship is an issue i haven't sussed out. an idea my friend had was to put it on a crane attached to the mast, and i had the idea of having an option to equip it on the back when your at port at the cost of loosing your rowboat slot. Both don't seem the best idea's to me personally.
