I'll have to insist. We need water. Plain, simple water.
Space engineers is a game made for people to create stuff. What we have to interact the stuff we create with is basically the environment. Right now what we have are planets, asteroids and the empty space in the vacuum. The only practical difference between planets and asteroids is gravity strenght and environment size. Atmosphere is great, but unless you use mods, there's no wind, so there's little difference in engineering to asteroids and planets... it just adds a layer to resource management with different thuster types.
Now, if you add water to Space Engineers, besides land and space, we will have a third major environment to create stuff for. It really doesn't need to be spectacular water. It doesn't need to be volumetric. People don't want carry water around to make pools in their bases, but I'm certain they would want to make underwater bases. None of the environments we have right now are spectacular in regards to simulation because: a) voxels aren't exactly realistic and b) space... well, it's just space. What really makes these environments spectacular are not the environment themselves, but how we and our creations are able to interact with them.
Imo water could be just a fixed level present on certain planets. Dig until ocean level and you get to the water. Simple water like that is not hard to implement. You already have air tightness mechanics. Rename atmospheric thrusters to something like displacement thusters and add a dehumidifier block to clear interior grids. I know the harder part is implementing buoyancy but even that does not seem very complicated. It's basically just a reverse gravity field that also takes into account air mass inside the grid. Would it have a significant impact in the game processing? I know there are a lot of nitpicks involved but we just need a basic version and modders can implement more realism if they want like there is with atmosphere and thrusters physics for example.
Imagine you need to build a ship airtight and heavy enough to dive for resources. Imagine building a base in a planet where the whole surface is covered in water. Imagine launching a rocket to space from under the sea. Don't waste that.