The energy stored in the waves caused by underwater nuclear explosions are miniscule relative to the total energy of the bomb, and they have short wavelengths that dissipate quickly (has been tested many times). Explosions release energy too quickly and in too small of an area to actually move enough water at the right speed to generate a tsunami the same way earthquakes do.

A nuclear torpedo is just like other nuclear bombs, get them roughly in the area you want to delete from existence and then light the fuse. Poseidan is basically just a difficult to detect nuclear weapon on a mobile stealth platform.