My concern with using a hammer or knife to break a window in a vehicle filled with water is the water resistance impeding the blow when striking the window. The Res-Q-Me or center punch is not affected by water resistance (of the strike) and should make breaking the window easier than with a hammer or knife. The hand/arm only under water is not taking into account the water resistance against the whole body. Think of it this way if you stand above the water surface and punch water, your body is anchored on a firm surface, now go underwater and punch, your whole body will recoil slightly losing power.

Pete