Leave it alone while you are still in the water.
Many in this situation have applied "just a bit more force" with addtional assistance and were greeted with the sound of water GUSHING IN.
Run, don't walk, to your nearest marina and buy a couple of DC plugs; tapered wooden plugs. If they don't have them, you can get them from Defender
in 3-4 days for 9.99 plus shipping
Have one stored near the valve - tied with a piece of light line. Have the other someplace in the cabin
where it can be found in an emergency
. Make sure you have a hammer (preferably a deadblow) handy as well.
If your hose lets go, you can use this plug
- tapped into place - to stop your boat
If you are in a warm-enough environment
, you can go below the boat and tap a plug
into the through-hull to allow you to take the damaged hose off inside without being able to move the valve.
This should be fixed but, if you can't haul for now, you should at least buy some plugs and piece of mind.