Need Help for an Eldritch Naval Project.

I Recommend A Fleet of Ships it would add to the Life of the campain the voyage of the unending sea was a campain run by DM Mitch (I think) the goal was to find the end of the world.
Whatever your campaign ends up like the goal should be to get to an important location for the campain maybe they are explorers or they could be headed to an important location for a specific item to save the world.

This sounds like a lot of fun!

I think that naval based adventures set around an Eldritch Horror theme are a great combination, mostly considering the Deep Dark of the Sea combined with the Hidden Evil of the Old Ones. Where better for them to sleep for eons than at the bottom of the murky ocean, far from the eyes of humankind, and only made mention in the mad scribbles of insane sea folks.

For your boat idea, I think that could be taken out longer than a one shot, especially if the focus was on isolation based horror. If your Party are trapped on the boat, then wierd things are going to be more pressing and dangeorus, and "Cast fireball" isn't necessarily going to be the best outcome. Maybe some of the sailors are part of a Cult, and they are taking the ship and it's occupants out to the deep sea to become sacrifices, but this takes time. Or maybe the Old Ones are slowly turning those onboard mad, one at a time, or making them kill others or sabotage the ship. Or maybe the Party are the ones infected by the thoughts of the Old Ones...

These ideas could tie in with the island, which could be known as a final destination and mega bad place, or could be part of a potential shipwreck/saviour sitation, in which it is seen as a place of hope when all was lost. Though will then obviously still be a mega bad place but will take them slightly longer to work that out. Either way, maybe its up to the Party to survive and escape, and that'll give you another sea voyage to turn into a chase or something high intensity. And then they're home, in civilsation, and safe. Except who is that strange looking man who follows them, and why does he look like the Cult leader/Ship captain who they watched drowned and wrapped in tentacles near a month before?

I am excited to hear what you come up with for your adventure!

I Love that whiskykuts.

For what I have now, the boat ride will not be as long as I would prefer, but I tried to keep it to the point. The player are gonna have cards with the information they know about each individual on he boat (I was thinking 16-20 people). The player are normal human with no special power, and are working on a merchant boat transporting an Ancien Artifact for a collectionner. The artifact originated in a small tribe who was for a long time believed to be a Myth until now. This tribe are protector of the unknown and somehow "linked" to the Old Ones. What players don't know is that there is one of those tribesman on board and things are about to go bad.

I want the "investigation" to end up by a sacrifice to the Old One provoking the inevitable end of the ship. The players are gonna end up on an Island where the unknown tribe reside. As they try to go back to the real world the Old One's influence is trying to make sure that none of them leave this Island alive... or sane.

I'm just taking the ideas above and running with them, but ...

For a longer campaign, I would probably start players as mariners on a ship (love the idea with cards for the crew, Fender) and then start with small "eldrich" threats from the sea: finding an empty fishing boat with just a single low-level threat, ramping up to a tentacular attack from the water, some slimy idol that needs moving from one location to another and undead/weird creatures gathering in its wake, a cult as medium level threat. The major threat of the campaign would be a sunken city rising up and a Big Bad with Its cthulhuesque servants (think R'lyeh rising from the deep).

At some point, I'd draw inspiration from China Miéville's The Scar, in which he has a pirate city consisting of numerous vessels of different kinds tied together and being tugged around on the seas. And there will have to be some underwater explorations and combat situations.