So, how are you planning on keeping things from being teleported on to you Spelljammer vessel? If you haven’t thought about it yet, you should. The spellcaster helming a Spelljammer doesn’t need to use any of their actions to maneuver their ship. This leaves them free to teleport all sorts of things over to your ship once they maneuver into range. Things like a boarding party, or a delayed blast fireball. You’re gonna need a dimension lock.
Your new friend, the Canaloth, can be found on page 69 of Mordenkainen Presents: Monsters of the Multiverse (MPMM).
A type of yugoloth, canoloths are fiendish trackers and guardians employed by evil powers. They prefer to enter into contracts to guard valuable treasures and important locations. They always do exactly as asked—never any more, never any less.MPMM, p. 69
Here’s what it does that you need:
Dimensional Lock. Other creatures can’t teleport to or from a space within 60 feet of the canoloth. Any attempt to do so is wasted.MPMM, p. 69
Of course, they aren’t very good pets. They’re always looking for a technicality they can screw you over with. Most of you are probably too good to get one of these things the food it needs to stay healthy. You might want to find another way to secure your vessel. Especially if you’ve got a ship big enough that you’ll need more than one of these things.
Dimension Lock Another Way?
Of course, there are probably other ways to protect your vessel. Here at the DungeonSports Colseum, we have more than a few extra canaloths lying around. Vexia has been cutting them up and putting different chunks of them in jars to see if she can isolate which parts affect the dimension lock. The Coliseum Complex has its own dimension locking capability, but those are mediated by the Teleportarium, and the equipment that does it is rather bulky to it out a smaller vessel with.
Mordenkainen’s Private Sanctum (Player’s Handbook, p. 262) can block teleportation, but the way it reads, I’d rule that it is a stationary field. Maybe a wizard who’s just as sharp as a tack can figure out how to alter the spell to protect a vessel?