"Run without them if you plan
"Run without them if you plan on doing a rebuild real soon.
I could try and explain it but will post a narrative from Joe Reeves, a retired OMC Master Technican - he explains it quite nicely:
Many engines are considered high performance engines and demand a thermostat(s), and many smaller hp engines also require a thermostat for the following reason. The pistons actually distort/twist off round when running. The thermostat is required to keep the outer wall surrounding the steel cylinder, and the cylinder wall itself at a temperture to compensate for the distortion.
With the thermostat removed, the difference in temperture between the inside the cylinder and the water jacket quickly reaches a point whereas (in effect) you have a oblong piston running up and down a round cylinder, or vice versa. Bottom line is without the thermostat, the piston and cylinder wall could be damaged in a short period of time.
The thermostat in your car isn't there simply to have the heater function properly..... think about it."
Now "SOME GUY" will always say "it's ok if you run in warmer water" but the operating temperature of most Merc's is 143 degrees, so the water you are in would have to be well over 120 degrees to compensate for the lack of a thermostat. Does that sound like the water temp where you boat??
Most times thermostats are pulled because the owner is too lazy/cheap to service the waterpump when it needs it. If the motor is overheating, pulling the thermo's will only buy a little bit of time until you weld the powerhead into a solid chunk of steel/aluminum."
A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)