Ultra, sorry to hear that your boat is still not fixed. TBI's rely on manifold vacuum to control just about all engine parameters. This relays engine load to the ECM. I've seen folks do engine swaps, and hook the vacuum line from the MAP sensor to "ported" vacuum instead of "manifold" vacuum. This would cause a hell of a drivability problem, and it's easy to do. The TBI has ports for both vacuum signals, and this would be an easy mixup for anyone. Not saying this is your problem, just something I've seen happen that pissed people off trying to figure out. Crank the motor up and pull the MAP vacuum line off the TBI, stick your finger over the port and make sure it's pulling vacuum at idle. Ported or timed vacuum only pulls when the throttle plates are cracked open. The manifold vacuum port is "usually" the lowest, or closest to the base of the TBI unit. something to check anyway.
fog