Now I am no expert on these matters but I believe the shape of the nose of the Hydros may explains their effectiveness.

Couple of points. Flat nose projectiles have a history of killing more emphatically and reliably. Elmer Keith even noted that the increase of the metplat by .001 seemed to kill quicker. The nose of the Hydros has a cup or dish in its nose. This may well increase the fluid(Hydro) shock wave by increasing it's force or shaping it. Hit any living animal hard enough in the chest area-blunt force trauma-and you can stop the heart.

With the increased shock wave a close to the spine hit may well send enough of a shock to the brain via the central nervous system to kill.

When a bullet goes through tissue it creates a permanent and temporary wound channels. Apparently the temporary wound channel "starts" to dissipate fairly quickly at the extremities. The Hydros might well make this temporary channel more permanent as well.

My father was a Civil Engineer who amongst other things specialised in Water. Some of the things that water or shock waves were used for or the resultant destruction/damage from shock waves was incredible.