Re: V6 Timing Engine.Setting idle in water,in gear question.
The difference is the back pressure on the exhaust, which is sort of a personal thing with each engine. Best bet would be to set it a couple of hundred rpm high on the cuff. Probably would be pretty close. If you can't do it dynamically, guess, adjust, and splash is the only routine you have left unless you have a professional testing tank and jacks to adjust the boat for natural water level on the engine.
In my experience, it really isn't a very touchy adjustment. As long as it is high enough to keep running and low enough so you don't reduce the diameter of your gears when you engage them, it should be good. Changes a bit with the weather anyway.