sell the boat and motor and buy a diesel.
E15 is going to kill a lot of outboards, the marine industry is not prepared for this and they are in panic mode.
Just kidding..but E15 will cause problems. I think any of the OB's out there today are pretty decent. Just make sure you keep your high quality fuel filter really clean (change often) and use whatever product is recommened by the company in the fuel. (I had a Yamaha 4stroke and used their "ring-free", and I can tell you it makes a hell of a difference!!, I would not have belived it but that stuff is magic)