I assume they are ball type seacocks?
Personally, I'd remove them and rebuild
them on a bench if they are in decent shape. You won't know until you get them apart. You can apply some heat to the upper portion to disassemble the valve. But do use Kroil or some other fine lube to soak into the threads as it's turning to prevent galling of the threads. Older valves use to have a bronze ball chrome plated. The chrome comes off and they seize up.
If they are a quality valve like a Perko you can buy rebuild
kits for them. I sand blast and wire brush before re-assembly.
If they are a plug
valve then they'll need to be soaked in some diesel fuel
for awhile. Then release the bottom nut and put some pressure on the handle and smack the side of the body with a hammer. But not too hard.