Player is building under caves
Since im a Owner (4 Years) I never saw someone building under a Cave. I dont know if its a new bug, but want let you know to take care about that maybe.
Example how it look like:
players just get under the water with a bug using boats and build there AFAIK.
In response to misticos ():
players just get under the water with a bug using boats and build there AFAIK.
But they can't build on water.

Merged post

https://umod.org/plugins/building-restriction

That must help you. Try 0.1 and 0.0
"Maximum water depth": 0.1​