1. Disconnect the hose that goes to the valve.
2. Hose clamp a piece of clear plastic hose in place and hang the other end from the overhead.
3. You can work on the valve and the water level will come up the tubing (if you ever need to know where the water line is outside this works too.
4. You may be able to remove the valve packing if it's packed (a lot of ball valves are not, which is why ball valves are insane for thru hulls). BUT if you get the valve free, you'll take water thru the valve assembly, seat, packing etc.
As for freeing valves these rules always apply:
1. Lube it.
2. Beat it.
3. Heat it.
4. Beat it while heating it.
Usually stuck valves will break free with some firm hammering on the housing, but a little propane heat goes a long way too. If the packing or seals inside are plastic (remember what I said about ball valves for thru hulls?) then heating it is a problem and contraindicated (I hate that word!).
If you take the safe route you will plug them from under the hull (or haul the boat). You can do it in snorkle gear with a standard wood plug from any chandlery (or from the glitter stores, West Marine, et. al.). No need to dive in a second time if you free it or not. A long handle screw driver or dowel down the tubing will push it out from topside.
If it's a copper-alloy fitting be careful about pouring corrosion inhibitor into it. Most are medium-strong acids and may take the copper out on the way. I'm not 100% sure of what corrosion would form on those valves anyway.
Valve exercise is one of the most important things to maintain. Despite what the AYBCWJblah blah blah says, GATE valves or other substantial valves are better for all hull penetrations. They can be maintained and lubed. Go on any Coast Guard inspected boat and try to find a ball valve on something important. It will never happen.
Be careful and good luck.