Not boat related but maybe you guys can lend a few ideas...
I have a '64 chevelle with a sbc 350. The motor came out out of a '93 suburban that my father and I tore down and rebuilt. Only modifications made was an edelbrock performer cam, edelbrock performer rpm heads, 1.6 roller rockers, rpm air gap manifold etc etc. We have built several motors without any issues; however, this time we're baffled! We broke the motor in @ 2k for 20 min, changed oil and everything seemed fine for the first 500 miles. Then the motor developed a slight lifter-sounding tap that would come on and off. Things we have done: rechechecked rocker arm torque, thought maybe we had a collapsed lifter so we replaced and broke in once more, changed out rocker arms. The tap seems to be coming from the back of my motor and seems to be more noticable when hot and and over 14-1500 rpm. I called edelbrock and the first thing they asked was if I had the hardened pushrods that were .0100 longer....yes I do have em!

A few other things that I have not checked that could I suspect could be screwed up: bent or cracked flexplate, stripped distributor gear. I made plexiglass valvecovers and each side is getting plenty of oil being pumped up. Very good oil pressure. What do you think it could be? It sounds like a lifter tap but the rocker arms are fine when I check it. Also, it's not a simple exhaust leak!

Very good point...I've bumped it over a few times and have inspected it somewhat but I don't see anything (very hard to tell though). I've been told that even a hairline crack can cause a tapping.

rebuilt a 350 once had same issue turned out to be bad mech fuel pump, not saying thats your issue but check it out cheap to replace.

I did not check the crank pilot for proper fit...I went out this morning, let it warm up and had the ol lady rev it up while I was listening for the ticktap. It sounds more noticable from underneath the car and at the very rear of the engine (on the passenger side). How do you check for proper crank pilot fit? I'll jack it up a little later and try to inspect the flexplate a bit more thoroughly.

It is not consistent. You can not hear it at idle...its only when the motor is above 1500rpm and even more noticable when hot and above 1500rpm. From what I've heard...a cracked flexplae will expand at higher temps and rpms and will thus cause a tapping? Any other takes?

Revision...Almost sounds like a knock compared to a tap. I'm seriously thinking it's something in the transmission area. Torque converter, flexplate....