Have a 2006 Crownline 225 LPX with a 350 mag, captains call, with Bravo 1 with the 2 point quick drain that is overheating at idle. Have replaced the thermostat, impeller, engine water pump and finally the whole raw water pump. Starts up, temp rises to 180 and all seems fine. After a minute or two the temp starts to creep up. If I rev it up the temp comes down to around 150. Have hooked a hose directly to the raw water pump inlet with the same results so don't believe its in the outdrive. It has the power steering cooler which is clear and no engine oil cooler so don't believe there is blockage. Have seen a couple of posts which talk about not enough water is going to the engine and some 350 mags need a restrictor installed to force more water through the block. anyone know more about this restrictor or have any ideas?* Thanks.
I once ran across this when I had my shop. This is worth looking at and may be the problem.
Look in the thermostat housing and be sure that the metered by-pass hole is not clogged up with corrosion, it may not even be visible if it is.