The C-Lion1 cable that was cut was reported buried 1 meter below the seabed.
Unfortunately, that wasn't deep enough since heavy ship anchors which can dig in deeper than that.
Probably impractical. Digging underwater is much harder than carefully lowering something on the floor.
There's already underwater powered trenching/laying equipment they use to bury the cables within X miles from the shore. The Baltic Sea is probably shallow enough to bury it all the way across at a higher cost per mile of course.