Please help!
Recently spent 20K with a reputable engine builder to rebuilt a 525 EFI. Upon receiving the motor back the water temperature climbs to 165 degrees at idle. If you increase the speed to 2000 rpm it goes back to 150 degrees. New sea water pump, new recirculation pump and thermostat have been installed and the temperature is incroaching 180 degrees now! Engine builder says new motors run hotter..... Mercruiser book says if it reaches 160 degrees take it to your dealer. All coolers have been flushed out, water pressure at idle is 5 psi, at WOT 25 psi. Does anybody have any idea on what it could be?

well first thing is the gauge right? will it climb past 180 or will it stay there? i would look into the sea water pump i had a few bad ones that were new i thought the water pressure should be alittle higher at idle .. good luck

It's a closed cooling system. So, also make sure there are no air pockets trapped in the cooling system. Also, make sure all the plumbing hoses are on correctly and system isn't hooked up backwards somewhere, including the inlet and outlet of the raw water pump.

and if it is stable at 180 at WOT over distance, there isn't a single thing wrong with that. operative term is STABLE

25 is a way too low , my 525 EFI , it`s 45 to 60 psi at WOT , you have dual pick-up drive ? look for obstruction ....
Regards
Christian