In general, the game needs more sea-based objectives to satiate player complaints of "emptiness" while in transit, as well as provide additional opportunities for two player ships to meet on the sea, rather than around an island (or in the worst/most annoying case, camping an outpost).
Hunting NPC sea monsters by expanding the Hungering Deep content is one way to do it, but also expanding on the skeleton forts concept by having NPC skeleton galleons and ghost ships to harass players at sea would help satisfy this issue.
Shooting cannons at NPC targets from time to time should be seen by devs as a way to break what many players see as the tedium of sailing without having to find, by necessity of the current design, the other players in the server.
Additionally, the factions could also benefit from having NPC ships at sea; the Order of Souls could task you with boarding and assassinating the captain of the ghost ship, the Gold Hoarders could task you with stealing treasure chests from the holds of the ghost ship, and the Merchant Alliance could task you with capturing ... chicken skeletons? ... from the holds of the ghost ship.
To reiterate as a general piece of advice, there just needs to be more of an NPC presence in the game, and more player beats happening in transit between islands/outposts, to fill the gaps when players cannot find one another on a server but want to do more on their ship than just sailing to a destination to fight skellies on foot.
