Most likely the shaft seal if it wasn't checked and adjusted on the water with the boat in gear (assuming its a traditional seal with packing). Should just be a few drips a minute while in gear. Only way to figure out where it's leaking is to do some looking at anything that goes through the hull below the water line while you're out there. Check clamps on any hoses/through-hulls to make sure they're tight.
With the seal adjusted properly, I don't take on enough water to make my bilge run on any given overnight.
Also, you'd get more help if you posted this in the boat section.