While putting some good pressure on the handle, tap the side of the valve with a hammer. But don't get carried away, depending on how it's mounted. A true thruhull valve can take a beating. But a screw-on could break off.
Another way would be heat it with the use of a propane
torch. Only heat it hot enough to still be able to touch and hold with your hand. Some ball valves have plastic seats that could be damaged with too much heat.
If these don't work
your next option, besides a haul out
, would be to put a plug
into it from the outside, pull the hose and full it with diesel fuel
to soak it for a day or two.