damn...that would give so many tactical possibilities
Check out my 2 maps: "Survival_Mayhem&BO_3d_v1" "Survival_Mayhem&BO_3d_RPG" as well as my mod: "Survival Mayhem&BO balance" -- let me know of any bugs or issues regarding those 3.
also i think ill have to be that guy and mention that terrain deformation is possible. I even remember there was some proof of concept sentons test that did it. Also there was some terrain deform mod that deformed things when you made explosions.
That said though this isnt quite what you might be looking for, but it could be used as a workaround if changing the water level isnt possible. The way is to then change the level of the entire map to make it sink into the sea. This may or may not cause loads of issues with things like structures.