Research!

Why? What should lead to that?

That depends on what happened. If the whole structure collapsed instantly, they are probably crushed by debris and a shockwave. But if there was a “leak” and the pressure equalized without complete destruction maybe the lungs are compressed, ribs broken and eardrums torn. All depending on the speed of equalization. Maybe also bones break (because water is compressible) but the bag of meat and blood should remain intact. So finding a body would help to reconstruct what happened. But I doubt they will find one before hungry animals do.

Please, it is not a war. It is a Special Operation!