You can get a pretty good effect somewhat easier than cutting out all those stencils. First, you need to reverse your thinking a bit. Start by painting the boat with the lighter color (grass). Once you've got the base color down, get a few handfuls of grass and hold them against the boat. Spray the dark colors over the grass and against the hull. You can mix the dark colors a bit to get different shading and shadows.
I feel slightly sorry for a man who has never patterned his gun, who has no idea how far his chosen load will retain killing penetration. But I'm extremely sorry for the ducks he shoots at beyond the killing range of his gun and load - Bob Brister