content: to build a 100MT warhead one needs to ask WHY.  The effects of a weapon detonated underwater are going to be very muted. If your goal is to destroy an area with a mega tsunami, then you'll need a far, far bigger warhead than for an airburst.  The first LAW is simply NO! NO vehicle designable by man can travel underwater for more than 6000 miles at 100 knots. Nope. Not going to happen. End of story.  If it really is nuclear-powered, it can travel as far as it wants as long as the screw holds up. It might not manage 100kt for the entire journey(especially considering the need to sneak by sonar systems), but it can almost certainly get up and go.  But other than this there just isn't a valid military reason to have such a weapon. Especially not one that by definition would only be detonated under water.  Almost every interesting civilian target is on a coast or large waterway.