[SgtHooper]

“...small nuclear warheads on torpedoes the sub does not have to get even close...”

That works equally well in reverse as the hydraulic pressure wave would be enormous and effective against subs.

[JAKraig]

Actually, not so much. there is a wave to be sure but the pressure goes up not down, it takes the path of least resistance so that the underwater pressure is not so great unless it is close. For instance, a 1/10th megaton bomb could take out everything within say a half mile on the surface because of the wave, the wave would knock over the ships. The vast portion of the energy of the explosion goes nearly straight up over the bomb, when the water comes down however it creates a huge wave on the water surface. That wave right after the wave from the initial blast will cause a lot of damage. There will be little or no heat or radiation damage if the bomb is deep enough except for ships pretty close to the blast, mostly just kinetic damage from water.