All of my boats uses one motor per one shaft/prop. I have several two motor boats, one three motor boat and one four motor boat.
The two motor/shaft/prop boats use two ESCs for independent control. (The current price for a Hobbywing 1060 ESC is about $24 (USD). As was said previously, you get much better control with this configuration.
For my three motor/shaft/prop boat I use two ESCs, driving the outer motors and the middle motor is turned on and off by a servo activated switch for a single speed forward (I don't see a use for the middle motor to go in reverse).
My four motor/shaft/prop boat (photo) is basically the same as the three motor configuration. Two ESCs, drive the outer motors and the middle motors are on a servo activated switch for a single speed forward.
I don't have any gears, belts, etc., only direct drives. The less you have between the motor and the shaft the less chance of anything slipping or breaking.