I'm not aware of a way to unstick a frozen "open" seacock safely, while remaining in the water.
Somehow you need to get a rust/corrosion disolving solution into the valve, how you do this, without removing the attached hoses and clamps I don't know. Maybe someone else does?
I would haul the boat even if it's for a short time, remove the hoses, loosen the valves, remove the stems, clean, lube and replace them. While you are at it, I'd check the hoses and replace the clamps.