25hp outboard won't run with water!

[Sackmanridge1]

I have a 1980 25hp merc that runs fine until you put it in the water. I start it up in the driveway (with muffs) but I do not turn the water on until after it starts. As soon as the water gets turned on it has a miss and won't idle. The motor needs to be almost 1/2 throttle to keep it running in the water. The carb has all been rebuilt. The only thing I can think is that it sits to low in the water and that causes water to be sucked in through my exhaust which is making my motor run bad. Any help is appreciated. I am new to this and I do not know a lot about outboards I just know the basics.