If you have a Forespar water strainer, make sure there is a rubber washer between the clear plastic lid and the knob that screws the lid down.
I had a problem recently where it would take a long time for the water to be pushed out the exhaust from a cold start unless I raced the engine. It turns out the washer was missing--perhaps I didn't notice it had fallen off after clearing the strainer.
One of the symptoms of a missing washer was the lack of adequate water flow through the exhaust at low rpm, sometimes leading to an engine overheat alarm. Once the washer was replaced, the raw water prime was established within seconds of starting the engine. Haven't had any more trouble with the water lift system since.
If you pour water into the canister, you might first close the raw water intake thru hull, then secure the strainer lid after filling the canister, and then open the thru hull. Don't forget the last step!
Without a doubt, fill the sea strainer with water and be sure the top is well sealed, unless you truly enjoy replacing your main engine impeller, often. It only takes a few seconds to damage an older impeller by running it dry.