First thing I'd do is attach the pump
handle to the emergency, manual bilge pump
to be ready for supplementing the automatic pump
. I would then locate the source of water entry and attempt plugging from the interior with the most appropriate material at hand.
We keep wood plugs at each through-hull, but for a larger gash, I would choose wrappping material around the plug - perhaps onboard PFDs, bedding, or a carpet. Using wood bilge access panels or cabinet doors are also options - secured with screws hammered or power-driven to the hull with my DC driver.
Very large holes could be wrapped from the exterior with a headsail or spinaker, to slow down water entry - secured by lines
attached at it's three points and lashed around the deck. I don't know how I'd attempt this, other than tethering myself to the boat and jumping overboard.
One thing's for certain, I would fight 'till the water entry is stopped, successfully overcome by pumps
. . . or I need to man the lifeboat.