I agree with the percussion method of breaking them loose. however no one has mentioned the fact that seacocks are a normal maintenance item. if yours are as bad as you say disassemble the whole thing and get some valve lapping compound, reassemble with the compound in place and proceed to operate the plug to clean up/true up the fit between the body and the plug. when your happy with the condition of the meeting faces, disassemble, clean everything thoroughly and reassemble, liberally greasing it with 3M silicone paste. tighten until you have to apply moderate pressure to move the plug. but don't forget they have to be greased every year.