At highway speeds, you should have plenty of cooling capacity so if you're getting the light on while at speed, you've either got a bad sensor or a serious issue like insufficient coolant, trapped ar in the system, or a bad water pump.
The cooling fan and the high temp indicator are on separate circuits.
I'd first flush the system and make sure you have good coolant. Then I'd look to the thermostat. Finally, the water pump. If you're losing coolant, the cap might be bad.
If the bike has no other overheating symptoms, it's possible that everything might be explained by the idiot light sensor coming on too early. Normally, the cooling fan comes on much earlier than the high temp light, so I wonder if it's possible that everything is fine - just the light is coming on prematurely.
I wouldn't mess around with water wetter right now. This is a band aid and would just mask the symptoms slightly.