If you gear it down it'll mean that prop will turn slower, will do less work, but let the motor do it easier/cooler. You could do the same thing by using a finer pitched prop. Same prop rpm as without the gearing, but less water being moved by each revolution. Or, keep the same pitch as now and reduce the props diameter. Just a few methods of doing the same thing, some less expensive than others. Then again, it depends on what the boat's purpose and hull type are. A reduction in prop speed, or water moved each revolution of the prop is sort of counter productive for a 'go fast' boat, not so bad for a slower one... sort of.
Water cooling the motor is another solution, depending on just how hot the motor gets. Would you like to narrow the playing field a bit and tell us what kind of boat you have so we can make more guesses?
- 'Doc