I am wondering if I check my oil in the water cold how much is on the dipstick ?

Cuz level in the driveway is diff ..

It will vary all over the place depending on what the boat attitude is when you check it. Even then, some of the dipsticks are plain wrong (do a search here). Best course of action is to fill with the reccommended quarts, run the engine, allow to drain, then mark the dipstick for the level on the trailer and in calm water sitting still. My water reading will change quite a bit if we have extra people or equipment in the boat.

Is it possible too burn a quart in about 15 hrs of use, cuz it seems to vary alot. I get about 1/2 on the dipstick between add and full. But I guess I will do oil change and do like you said. What is a good oil someone told me 25 w 40 merc is good. What do you use ?

A qt of oil every 15hrs is normal with a lot of Merc engines. My 502mpi's use that much oil.

My dipstick shows about 1/2 qt more when its on the trailer on level ground.

A qt every 15 hrs is definitely normal. I wouldn't be surprised if it burned more than that.
If you have a drain hose in the bottom of the pan, then drain all of it out and put exactly 8 quarts back in. Then check your stick both on the trailer and in the water if you can. You will see a slight diff.
Eddie