Some boaters, rather than to spend a few dollars to replace a failing water pump will remove all and anything blocking the water passageways in a mistaken belief that will cure the overheating problem. Apparently your engine is in this condition.
(Thermostat Purpose)
(J. Reeves)
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 temperature to compensate for the distortion.
With the thermostat removed, the difference in temperature 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.