If you need a spelljammer that also serves as a submarine, the turtle ship spelljammer is the vessel you are looking for. It is also painfully slow. So slow. So very slow. Slow as a halfling, but still a giant boat spaceship. I’ve done an analysis of the attributes of spelljamming ships, and the speed of this vessels stounds out so much from that of the other ships that I’ve assigned a trait of submergibility a huge penalty to speed.
The only other vessels that have a similarly large penalty are those that are marked as ancient. I have to presume that this relates to the fact that newer, more powerful magics have been created with relation to spelljammer helms. There are other ships that are inexplicably fast. I’ve drawn a similar conclusion about certain ships that move faster than others that are otherwise identical; These must occur as a result of more advanced magics used in the creation of the spelljammer helm.
Story and the Turtle Ship Spelljammer
The turtle ship allows for a great deal of verticality in the story of your game. Your players will have a simple means of traveling from the depths of the sea to the sky, and then out into space. Maybe after all that, they’ll want to visit the elemental plane of water? This ship has them covered. This will also give the DM a lot of variability in settings combat encounters. Players have a starting position for encounters under and on the ocean, in the sky, and in space.
I’ll be back with more good stuff for your game. In the meantime, I’ve made token/maps for all of the published spelljammer vessels in the AAG. If the turtle ship spelljammer isn’t for you, check out other spelljammer maps.