I'm guessing electrical also. Probably the main harness connection to the engine. I've also had corrosion on my ignition module plug in that made the boat run like crap.

I have found in my kind of limited and certainly frustrating electrical troubleshooting exp. " All Roads Lead to the GROUND! I have been bitten on many accasions chasing problems around in circles only to find a bad ground connection somewhere.....even knowing this I did it agian a week ago on my trim pump...took the whole thing apart checking brushes and looking for burnt windings..nothing..jump it across a battery on the bench ....it was the stinking ground connection in the boat....Go right for your grounding cicuits first ....esp. when electrical goodies are acting wierd...

and what Griff said

Thanks guys! You're all confirming what I suspected. Something in the electrical system most likely failed. (Considering that the darn cable ties that hold the main part of the wiring harness up always pop loose, I figure something got broken or knocked loose from the thing bouncing around half the day.) However, the weird metallic noise that happened first could be something else.
Turbojack - Parkside is getting first crack at her, simply because she's there. If that doesn't work, I think I'm going to try Mars Marine. I can limp her over there without messing with the trailer.

Belt loose on circ pump and alternator?
Might explain overheating (which would also cause spark knock under load) and weird electrical oddities (as batteries dropped in voltage).

While it was acting oddly, did the trim system still work okay? Was you battery selector switch in its usual position?

Just throwing out stuff to think about.

She didn't overheat. The water temp gauge just went nuts (mostly it read too low - like around 100). One of my friends confirmed plenty of water out my exhausts and I still had normal water pressure. Trim was fine and with only one battery, the switch has only one "on" position. Just waiting for the mechanic to get to her and give me a report.

Update
 

Well, it's not just electrical. The mechanic found water in cylinders 5 and 7. (No wonder she wouldn't plane.) He got the water out, oiled the cylinders and is going to pull the motor and check everything. Could be as simple as a head gasket or something more. Thank goodness the season is long down here.