Not quite clear on the question Dyno readings are usually in HP, when you talk PSI and outboards you are usually referring to Cooling Water Pressure. 12-15 PSI is as low a water pressure value as I would ever run a Mercury at WOT (wide open throttle) for any length of time. Maybe water pressure is not up to snuff and worried that overheating will occur, he is smart not to run it. Low H20 Pressure readings are a whole nother issue. if the motor is out of the water it could depends upon the water inlet source, ear muffs water tank? Or maybe the waterpump impeller has taken a set and isn't moving water efficiently or its not working at all. You're not gonna see real high pressure until you give it some RPM. Good Luck and don't dyno a fresh rebuild, let it break in properly first.