Electric Motors are a great way to power a model boat. Especially if you need a boat that is both quiet and super portable. To extend the life of any electric motor, be sure to run a cooling coil on the motor to remove unwanted heat generated from running your boat.
Electric motors are also very "torque'y" and can turn high RPM very reliably. These motors also range in shape and size. Some boats use two motors driving a gear box to drive bigger props. Starting at a $20 Stock 05 motor, going to a Graupner Turbo 700 at about $40, all the way up to $200 for an Astro Flight 05. Motors are available in different voltages and there are also 3 sized "cans" 550-size is stock (05 size), 540-size is "Pro".
Then there is Graupner Speed 700 size of 1-11/16" diameter and a 5mm output shaft. This motor is available in 9.6V or 12V for the modelers looking for all out performance and top speed.