Just wanted to chime in here again, there's quite a few possibilities and many have been brought up in the past from what I have seen some of the better ones include:
Simply providing a proverbial carrot for players to repeat content will make it easier to find others whose answer to helping won't simply be, No. Sure social development, friendships, fun would be great if that were enough. But even my friends usually pay for gas if I give them a lift to the store.
Friendly outposts or similar have been mentioned both in this thread and elsewhere. Now, these outposts need not be friendly, but having a major outpost where the basic algorithm of play centralizes this location as constantly visited will help to force interaction with others for good or bad. But simply by having a place where others will be found will help mitigate the inability to find others to help in varied events. This could be done easily in game by simply dropping the Bilge Rat NPC into ONE area of the game instead of one on each current outpost. Players interested in the Bilge Rat for reasons of information, shopping, or even taking screenies with the drunk will encounter players with an obvious same interest. Heck, even a NEW island that's larger than the others with multiple ports, a mega mall with all the items from all the outposts, and so on... a central hub... would force players to meet. Sure, it'd be a great place to sink others and be a meanie... if it's a large enough place... that issue could be mitigated. Besides the quests already force us into proximity with other players so keeping us at a safe distance isn't the goal anyhow.
Simply not having multi-crew events is a solve. I have to admit, even I prefer simply needing more players ON MY SHIP than needing to find another ship altogether, but that flies in the face of the current desires held by Rare so it doesn't seem like much of an option - but it is an option.
Having a bulletin board in game that is server specific with messages that live only as long as a given player (the poster) is on the server works to aid in this issue as well. If I can log into a server, hop over to the bulletin board, read a request for help and commit to it by finding that player, my life just got 1000 times easier. Sure, finding that player might be a pain but at least you know they're on the server. It'd be better then if players could join a multi-crew party in game where each of text, quick-chat, and voice chat became linked and a new color for names appeared overhead (let's go with Rainbow hehe) so now everyone in this party can talk despite distance, identify who is in the party and who is not, and if rewards are on the line can even tell the game client who to reward. This can be attended too with Xbox party chat, but taking it a step further by helping players identify who is in their 'larger party' is potentially of value for many reasons. Simply being able to respond to a players BBoard message in game or with the xbox app could be of use, it could even allow the 3 voyages to be shared and larger voyages to be required later in the game which demand larger parties.
Increase the damn game world size and players to server count... I know... literally world breaking... but man the landscape is small for an oceanic domain... but that isn't really a fair request, sure if SoT was a procedural generated game similar in scope to Eve online with no size limits or players per server cap... the world would end.. but hey, I'd buy that game lol - obviously not a reality possible here.
There's more but these are the best fixes I have seen, thought of previously, or can think of right now. There are other options but I find them to be somewhat game breaking. Such as peaceful outposts.. obviously not something we want as it robs us of the freedom Rare so values we keep. Finding players outside of the game and bringing them to our server only leads to "server alphas" and "friends only" servers and that's not a good thing either, and so on...