If this is a fiberglass
boat and it is leaking through the fiberglass
and not around a thru hull
or a water leak from your fresh water system, its not likely a single
crack that you can squirt something into. Any topical coating will surely fail. For a large unknown spot I used roofing tar and covered it with a plastic bag. Held for a day till I had it hauled, sure was a pain to clean off. Turned out to be a thru hull
that was just imbeded into the fiberglass that cracked a large patch around that pipe. Looked like it was just letting water soak through the fiberglass. If I had just put a wooden plug
in from the outside it would have stopped the leak. Once hauled out, the thru hull pipe just came out in my hand with a foot wide circle of fiberglass around it. I ground the edges back glassed over the hole with multiple layers of glass and then redrillid the hole and put in a propper thru hull pipe with a mushroom head
a proper backing board flange adapter and ballcock valve. I can stand on it now. I cant figure out why a boat manufacturer would just stick a bronze nipple into but not through the fiberglass side. If it was leaking through the glass I would suspect a larger impact that crushed the fiberglass and it is seping through the crushed layers insted of an isolated crack. The repair would be to grind out the effected area and relay the glass over a much larger area than just the hole.
If you just want to stop the leak without hauling the boat anything from a golf tee to a 2x4 drove tightly into the leaking spot may slow it down, but anything less than a haul out
and a real repair will be just temporary. I have repaired leaks on several boats. Most were either something leaking from inside (water lines) around thru hulls chainplates or vents. Then crush damage from dock
other boats or logs
. I'm not making light of your problem and sorry for being long winded.