Ship is just an exterior location: "High Rock sea coast/Your Ship" (case important). Until housing is properly implemented, just stick with the default ship in that location and assume player always owns a ship. We can worry about the small/large ship once banking is implemented. This is scheduled for 0.6 on Roadmap
along with player housing.
For the basic setup, you just need to teleport player to that location and use the local Start marker to position player on deck. I can help you get this setup if you wish. Look to how fast travel uses Streaming World to move player to new locations and position them outside city gates for an example.
You will also need to serialize player's prior location in world (map pixel and position) before teleporting to ship so player can return later if they save on ship and exit game.
As player housing not implemented, dropped loot containers will not serialize on the ship like they do in classic. This is one of the reasons I have the ship scheduled for housing features in 0.6. There's a lot more I wish to do around this and I won't be up to this until the allotted point in roadmap.