"WOW! I don't think I'
"WOW! I don't think I've ever seen a Mercruiser with anything other than Rochester. Maybe it was an attempt at fixing their problem with run-on, which was pretty bad on the 4.3L Vortec motors. Still a good motor, though.
Personally, I hate carbs. They're a cheap, easy way to get gas into the engine but I don't like the problems. You might check into shopping around for the sensors, harness and ECM from car parts suppliers. Standard TPS (non-Corvette), MAP, ECT and knock sensors, Delphi ECM (MEFI 4) and harnesses are available. The tricky part will be getting the right program for it, which won't be from SuperTuner, etc. You may be able to get an ECM with a Chevy pickup program to use as a baseline from GM performance parts. Call around to see if any speed shops near you work with TBI systems and if they have the hardware, software and a technician who can acquire data, analyze it and make the proper changes for your application. I know someone in Indiana who can but it wouldn't be cheap. His shop is usually busy doing development work for GM, Delphi, MasterCraft and Indmar.
Another option would be to find a scrapped boat with a good drive that has the TBI on it. That way, you would unplug the motor from the boat harness and replace yours with it. If it's a Mercruiser, the motor side of the harness is the same pin configuration, but I would still check around for a diagram. You would strip your motor harness off, install the sensors and install the harness from the other motor. The program should work for your boat if the donor boat is a similar size and weight."