That appears to be a Blakes tapered cone. The best way of removing a frozen cone requires tapping it back in towards the boat from outside. The cone should be moving away from/out of the tapered valve body. Banging on the handle end only serves to drive it deeper into the taper and make removal
even harder. An oak or maple dowel sized to fit snugly inside the cone, and distribute force to the underside of the head
of the cone, works well.
Please do not attempt to free this in the water
. If you break it you'd be in a real predicament...