The OP is in long island, I don't think he's jumping in the water anytime soon.
Better work..fast..and have the right size hose....if you're going to pull one hose off and put another one on.....
I wouldn't be Banging on my seacock...while my boat is in the water..no matter how solid it seems.
If it doesn't loosen up with backing off a little on the nut and some light tapping...maybe some heat from a glue gun..I'd leave well enough alone..and catch it the next time you haul out....why risk making the situation worse?
Those Bungs have a lot of surface area...if they have been neglected for a long time...they'll need some lock-eeze ..stew..tap...more lock eeze...etc..
The bungs should be removed...cleaned with a lapping compound, lubricated and reinstalled......tapping out the bung with a wood block will keep the threads from getting buggered up...
These aren't ball valves..this is a tapered bung..
Gate Valves..for thru-hulls....no no ...not on any vessel of mine....
been there done that...