Re: problem here help
a 12' boat is a one or two person in-shore calm water boat. with a 6 it is a slow boat, but it will do inthe right circumstances. be sure you have oars or if not, paddles.
changing the transom height or prop is a waste of time and money.
If you put a 15 on it, it will still be a 12' boat, too small for open water. With the 15 you may be tempted to head out on a calm morning, only to find yourself in real trouble with the slightest increase in wind. Plus you will probably switch from a 3 gallon to a 6 gallon gas tank, adding weight.
your 6 is perfect for your 10' boat, but for the same reason you don't want to take that out, the 12 is basically the same.