260 Mercruiser has no power running rough blowing smoke

by Ben
(Perth)

I'm having a problem with my 260 mercruiser. It wont rev up, has no power and blows smoke. Out of the water it runs fine. I put it in the water and its okay for 50m then wont even get up on the plane and starts running rough. I take it back out and it still runs rough. Idles rough but then sometimes its okay. I have put a rebuilt holley on. Done plugs and leads and timing. A compression test shows all eight at 150psi. I had the carby rebuilt again and was fine in the water for 2 mins then same again. The only thing I haven't done is the coil and electronic module in the dizzy. Also can dodgy battery wiring do this? If you could please help with any info would be great. Thanks