Mercruiser 3.0L What Now?

My 1994 Armada Marada bow-rider with a Mercruiser Alpha 3.0 L has been an absolute blast for the five years I have had it. Aside from general maintenance (new plugs and wires two years ago) it ran fine until...

...hit the ramp two weeks ago and it wouldn't start. Trip to NAPA and replaced the ECM. Wouldn't start. Trip to NAPA again and a new coil and it ran beautifully. Had to manually tune, but ran through a full tank of gas with speed and engine sounding so perfect for another weekend. Then I put a fresh tank of gas in and sped away from the pump only to get a few hundred yards and it started backfiring and chugging. I thought the distributor cap/rotor slipped out of tune so I readjusted to a beautiful idle sound and tried to speed away with the same chugging/backfiring results. I fine tuned over and over without any change....stalls at about 2,500 rpm and then starts backfiring.

I don't mind replacing or repairing anything, but I'm scratching my head. Why would it idle so beautifully and then do nothing at full throttle? Is this plugs? The only thing I do notice is that the alternator belt is worn and loose (squeels at low RMP) with no room to tighten anymore.

I'm many years out of high school shop class so if you think I'm ignorant you are probably right.

Personally, I would not have started replacing things with the ECM. I would have inspected the cap and rotor first. I'm in salt water and never get more than 2 seasons out of a cap/rotor. With the new "ECMs", engines can go from running fine to crap quite quickly with further degradation of the cap.