Why don't boats have gears?

This is really simple, a transmission is simply not required unless the speed is really high.

The reason is that at lower speeds the propeller can slip more and at higher speeds it does not. What happens is that the prop slips, and it generates more thrust, just like having a transmission with different gears. This lets the prop generate more thrust, essentially giving you a lower gear ratio. At higher speed the slip is reduced and the prop acts more like a wheel, with a the prop giving a more constant advance ratio. The idea that a prop has a constant advance ratio (that is requires the boat to move a fixed distance forward for each revolution) ignores slip, and slip is always there, and the faster you go the lower it gets.