Why does everyone assume the need for submersibles? Why not a small patrol boat dropping some depth charges on a time delay in the middle of the night?
That's also an option, but you would have problems with precision and deniability*.
You could use some WW2 style device and blame it on German mines that are still present.
How else could it have been done.. What about explosives being built in from day 0? Sort of like how some bridges in Europe were pre-mined during the cold war?
This is sabotage for sure.