I have a 15ft Grummen Sportboat too!
I bought it in July 2007 and used it about 15 times last duck season. I paired it with a Johnson 6hp short shaft and it works very well. You need a bit of weight up front when you put a motor on it as the bow wants to come up. With full combat load it still clips right along. I have had 4 decoy bags, 2 guys, guns, blind bags, and all sorts of crap and a dog no problem. Pretty stable for what it is.
I put the camo job on it using Styx River stencils from Cabelas. This off season I need to rig up some sort of blind. Not sure how to do that yet?