Why not a submarine? We have a car (sort of), we have rafts, motorboats, and even a mek! So I've come up with some ideas that can make having a submarine useful but risky and well balanced for pvp and pve.
1. At least the size of a mosa or smaller.
2. Can only build within the sub. only certain structures allowed. Mainly only storage boxes and crafting structures. No foundation or wall like stuctures.
3. Creatures deal around 35% less damage considering it's metal. Dunkles however deal 25% less damage to the sub.
4. Tusos are able to grab a hold of the sub.
5. Around the speed of a raft.
6. Larger underwater creatures can target the sub from a much larger distance.
7. Can NOT go inside artifact caves.
8. Has the ability to shoot two torpedoes from either side of the sub. Does around the same damage as a rpg. But travels through water at a much higher speed considering the subs vulnerability slow speed and turn radius.
9. Has a spot light around the bottom front of the sub to see a bit better.
10. Immune to electricity.
11. Has a radar (NOT like tek helm) you must look at it when not operating the sub. Can only detect large creatures. Examples can't track Meg's but can track dunkles and anything larger.
12. Requires gasoline but unlike the motorboat, the sub consumes gas twice as fast.
And that's pretty much all I got currently. I hope you think about adding this. I fell in love with this game the second I laid eyes on it back in 2015. It would mean the world to me if I was able to make the game even just a tad bit better. Thanks wildcard for this awesome game.