I have a 1975 tri-hull that I recently bought and noticed that after the first time I had it on the water, I pulled the drain plug and about a gallon of water came out.  This is a 16-foot boat and I was on the water for maybe 4 hours.  There are no obvious leaks in the hull that I can see.  What I did notice was that the paint is worn beneath the front "points" of the hull and that you can see the fiberglass in the worn areas.  Also, the previous owner sealed some hull penetrations with household caulk.  First, how much bilge water is to be expected for a day of fishing and second, where is this water coming from?

If you can see the fiberglass under the hull and this area is under the waterline that's not good.  Fiberglass will absorb water and will evetually delaminate.  You could also have soaked foam flotation in the flooring and the water you're seeing is the drainage from the foam.  You could also be leaking from engine mounting bolts, hull drain at the transom or any livewell plumbing you might have.  

Thanks Cart:

The previous owner put in new deck carpeting.  In your experience, do you think if I pull up the carpeting I will find a hatch that will let me see into the area between the deck and the hull?  It's a '75 Lund tri-hull.

Normally no.  The previous owner should have installed the carpet over a solid floor.  I would seal off those areas under the hull with exposed fiberglass with a refinish gel coat refinish kit.  Overtons.com is one place that sells a bunch of refinishing stuff, so does iBoats.  Then I'd try putting the drain plug in and filling the hull full of water with a hose and see if you can find any type of leak that way.  After that, concentrate on any thru hull fittings and lastly, the livewell system if the boat has one.