Many times happened with me that i run into the shallow water and i can't start swiming fast enough.

I suggest a feature which allow the player to plunge into the water with the "step ower" button or action appear in the action menu.

This animation should holster any weapon and "move" player forward and under the sea level. To awoid glitches this should be allowed only if the player in the shallow water or near the water(ship dock) and at the "animation end position" is deep enougth the water.
Some indicator should notify the player that it is allower to plunge.

The side effect of the plunge should be that the AI be able to detect the plunge splash sound.