A technical hurdle we have with ships is the memory footprint they add to the game. With all the unique pieces on a boat, just by introducing a new type, we end up adding a lot of memory and there isn't really any other gameplay aspect that's comparable to it, once we make some improvements and overhauls to this system, it'll become a lot more likely that we'll be able to add additional ships, not to say that it won't happen now, but this is one of the things we have to workaround. Memory is an important aspect as it's something we need to consider when getting ATLAS to launch on console later on.
It's why in the past when we have decided to do introduce an update that effects ships, it tends to be based on introducing mechanics that work with ships, so think ship weapons/structures, their attachment system etc: Racks, Ballista Harpoon, Fishing Net.
The attachment system is pretty robust, so it wouldn't be surprising to see new systems based around it.