The load placed on the engine by the prop is a combination of diameter and pitch. If you want to add a 1/2" to the diameter, without changing the load, you'll need to compensate with less pitch.
That 1/2" larger prop will have more blade area, thus a better grip on the water. This is good at low speeds for better acceleration, but depending on how fast you plan on going, that blade area is going to increase drag, making it slower at speed.
Bottom line, props are a compromise.
