I changed out my water pump on 2006 115 EFI Mercury and no tell tale sign when started with muffler hooked to hose.
After reading other threads on this topic, I decided to buy a big 2' X 3' Behlen water tub and put the lower unit in water
and started engine and there was the tell tale stream of water. So these pumps need to be primed. That said it appears that when exiting ocean
and wanting to purge saltwater, the engine is still primed and can be used with water muffler as this was my experience the first time after putting boat
in the bay. But if you wait long enough the prime will disappear.
After reading other threads on this topic, I decided to buy a big 2' X 3' Behlen water tub and put the lower unit in water
and started engine and there was the tell tale stream of water. So these pumps need to be primed. That said it appears that when exiting ocean
and wanting to purge saltwater, the engine is still primed and can be used with water muffler as this was my experience the first time after putting boat
in the bay. But if you wait long enough the prime will disappear.