This just happened to me. I also noticed some problems with the islands and how they appear. I'm thinking it has something to do with the draw distance and those clouds on the horizon that hide everything.
Well here I thought I was starting to make my bases too big. Although I will say I try to keep mine within the realm of the possible-without-cheats (even if I use nocost).
Just happened to me. ONE of my seamoths is currently stuck at 6m. But somehow not the other 3. I noticed it after I used the teleport on the cannon Island (the one in the caves that leads to the floating island). It may have happened before though, …