My old 9.8 Merc has a thermostat just before the pee hole... have a peek and see if you have one. Could be all it is... at first fire the warmth of the day had it open... then cold water thru it closed it back up.
Other option... was the impellor you took out beat? Had a marina change my Honda impellor in 2005.. and it pee'd worse afterwards than it did before the change. Small pieces of rubber stuck in various spots in the system.