How old is the motor, fresh or salt water usage. Remove the thermo, if severely contaminated, rusted, salted whichever install a new thermo. If still overheats at any throttle setting seems all the internal water pasages are near clogged and with much less diam to disipate heat from the circulating water fast. To cure such awful scenario wll need to remove the cylinder head, exhaust covers if having and the powerhead from the base to expose all the water passages and clean them all for motor to work as when out of the box, only possible by mere hand mechanically speaking.
If the water pump it's pumping right, the grommet is in excellent state and remains hooked in place, the water tube is well connected to the lower water pump's grommet and upper one but keeps overheating you know what's needs to be done. Lower pic corresponds to a contaminated exhaust cover area, now imagine the cylinder head area condition...
Happy Boating
