Direct drive is the most common drive train type for entry level RC boat. There is no gearbox and the motor drives the propeller directly. There is a motor cooling coil formed with thin copper tube that carries heat away through water flow. Because there is no gearbox, the motor is taking heavy load.
When replacing the motor on a direct drive RC boat, torque is more important. A stock silver can is 27T.
If you pick anything faster than that such as 21T or 19T, the boat will go faster but the motor will get overloaded real fast. A better choice would be a standard 540 with strong magnet. Strong magnet offers way more torque. And it has a cooling fan inside the motor for better dissipation.
Do NOT use anything faster than 21T on a direct drive system. Doing so will damage the motor and the ESC real quick. The one shown below is a 9T. Motor like this fast requires gearing to produce reasonable torque!
