Careful application of measured force.
Groco added a 1/2" square hole to its flanged ball valve sea cock levers so a socket wrench handle could be applied for extra leverage. The company did counsel applying the additional force in a careful way, because it is possible to break the handle before your overcome the stiction and the ball rotates.
I've successfully used an old box wrench (an old spark plug
wrench) to be a helper.
Most all ball valve seacocks have drain plugs, so all water
can be drained from the seacock (for boats that are overwintering). If you remove the drain plug
, you should be able to squirt in a PTFE lubricant or a spray lanolin-based lubricant.
Some ball valves have PTFE or other linings. I'm not clear if what solvents (WD40 etc) could be used.
As you know, working the seacock once a month disrupts calcium scale deposits and marine