Like Phil said..the water type is extremely important.
A 15 is more than enough..by yourself..could be too much. Myt son has a 15 Yamaha on a 12 foot.I drove it..It's freaky scarey..yes it will plane out big time..he does better if you can get some weight up front..The 15 will weigh just over 100 pounds..
Here's my son's G3 12 footer with a 15 Yamaha on it..planed out.