[portofcall]

Think more in terms of killing a whole carrier battle group in one shot, or closing a major port (for a long time) due to blast damage and radiation. Larger bombs are diriter bombs, and detonating in water is pretty much the dirtiest use of a dirty bomb.

In short, this is a strategic terror weapon, an area denial weapon, and a radiological weapon.

[cryptonector]

Yes, quite. There's no tactical or even strategic value to this other than as a terror-based deterrent. Of course, all nuclear deterrence is terror-based to some degree, but weapons that can only really be meant to attack populations are more terrible than ones meant to attack military targets.

We saw this back during the height of the Cold War, where first very high-yield weapons were deployed due to lack of sufficient accuracy, then later these were replaced with lower-yield weapons as accuracy increased. These UUVs are a desperate move to make up for lower capability in other parts of the Russian deterrence strategy. They're scary indeed.

[caf]

There's no tactical or even strategic value to this other than as a terror-based deterrent.

Of course. It's explicitly a second-strike weapon: if it is ever actually used, it has already failed in its mission.

[marshray]

What makes water especially dirty for this purpose? It seems like more of the bad particles would be washed back out to sea than if they'd been dispersed as fallout dust.

[portofcall]

It might seem counterintuitive for the reason you described, but the problem with water and ground bursts, is that the entirety of the radioactive byproducts interact with large amounts of dense material. I’m a true airbursrs, the fireball doesn’t touch the ground, so the air itself is contaminated, along with material from the ground which is drawn up into the fireball through convention and blast effects. That’s still dirty, and in the case of the Tsar Bomba (50MT) catastrophically dirty. The total volume of affected material though is relatively low.

If you detonate underwater, all of fission byproducts, and unburned fuel comes into direct and prolonged contact with a large volume of dense material. In a shallow blast, gigstons of thst material are ejected into the atmosphere, while the rest remains underwater. It will be diluted, it a 100MT device is still going to produce horrendous and long-lasting effects.

That material will poison the underwater environment as it settles in the immediate area, and for km away as it dissipates. Worse, it will enter the food chain and be concentrated through progressive predation.

This covers the explosive (not radiological) effects of non-airbursts for some context! http://www.abomb1.org/nukeffct/enw77b2.html