I'd suggest thinking twice before using a hammer.
I've had too many bits break off when I used a little too much force.
I've had more success using heat.
My suggestions (Caution - the valve may break off completely. You must have an effect way of stopping water ingress if this happens. If you don't do not do this in the water.
1) Use whatever penetrating oil/fluid you think is appropriate after reading the label and getting technical advice.
I've had a bit of luck with silicon spray. Others argue that silicon has no place on a boat with good reason (It will be very hard to paint
anywhere the silicon goes. For "O" rings it's great).
2) Work the handle gently by hand. If you can get any movement at all then that's enough. Keeping the "cracks" wet with your lubricant of choice gently "massage" the valve handle.
3) Apply gentle heat from a heat gun or similar. Alternate heat/lubricant/massage - Use the cooling
after the heat to try to suck lubricant into the cracks.. If the valve is still frozen be prepared to up the heat a little. Keep the area well ventilated (If the area is very confined restrict yourself to only working a few minutes at a time with plenty of breaks to get fresh air.), and have a fire extinguisher handy!
In all of this patience is the key. Be satisfied with tiny movements, and be prepared to take days, weeks or even months to free the valve!