Sez DJ:...appears to have a large leak into the underfloor bilge. After about
4 hours on the water I have about 4 gallons or more of seawater ...

DJ, lots of information you left out...Are you outboard or inboard (or I/O)
powered? An inboard powered boat will uually have a small steady leak at the
prop shaft packing whereas an I/O or outboard powered boat won't, for obvious
reasons..Though an I/O can leak if the bellows is damaged... Does it leak at
rest (at anchor or in the slip?). Or only underway?..
Have a live bait tank? Sounds to me that you have a real concern, let us know
what you find.
Mike