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.
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