If it was running well on idle, out of the water - with the OPS carb, it was definitely too lean. A carb like that needs to be set so that it won't run below about half throttle out of the water. It will then clear out a bit in the water - due to the loading. I'm sure the CMB carb will make life a lot easier though.
Do check the plug. I won't even use the Model Technics Firepower plugs if they're given to me now. I had several myself, and know of more - where the seal has blown. Some of these even before the boat got in the water. In my case I ran a boat and it stopped suddenly. Brought it in, tried to start it - not much compression. Noticed the +ve pin in the middle of the plug was loose. Replaced it with another Firepower plug, and that one also failed in seconds. As mentioned, I know of others who had the same problem. I'm also pretty sure one of mine put shrapnel in the engine and caused some damage to the Piston / Liner. I since received a few Firepower plugs with some stuff I bought via eBay, and I just binned them - not worth the risk.
Do keep us posted !
Ian