Try soaking the fittings of the valve handle with penetrating oil first; let it work its way in for a while. Number one rule in plumbing: Never force a stuck valve as the handle will likely break off in your hand. But as the guys stated, it's probably due for a head rebuild if its that corroded. If that valve fails with the outer seacock open, it will flood your boat. Not worth the deferred maintenance. Another area where cheap windes up being expensive.