At a depth of 260-360 ft or less, any expert diver team could do this from the surface. It may not have been that high tech. Dive down, plant the bomb, come back up. And you wouldn't need that big a boat, either. I don't really know what a battlegroup sees.

I would also think that since submarines are hard to detect, the Russians could actually sneak up on a battlegroup and deploy saboteurs in the water.