Running without the thermostat temp control and the backpressure that it creates, the motor runs too cool and at low speeds combustion is not efficient so there is more carbon buildup on the pistons and rings plus the fuel is not burned completely as it should. Without the water pressure that the thermostat provides, the faster flow of coolant may cause eddy currents in the block and resulting hot-spots which can cause cylinder and piston scuffing.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.