overheats at idle

[capslovak]

i have a 260 mercruiser that overheats at idle. once rpms turn up to about 1500 engine cools to normal operating temp. also runs cool at cruise speed

water pump impeller in drive was changed, thermostat was changed. did not correct problem.

engine runs cool when flush (muffs) are attached to a hose on land but overheats when splashed

the motor has an aftermarket alpha I drive

thoughts about where the trouble could be?

thanks