I have a factory installed Marine Aire in my Fountain with the March P500 pump. It worked fine last year, but this year when I went to use it the pump would run but not circulate water. Today I confirmed that all passages are clear, strainer is good, etc. I can take a garden hose and water passes freely through the AC unit and backwards and forwards through the pickup and sea strainer, so no obstructions. I placed the pump hose in a bucket of water and it will not pickup or prime... however I can take the garden hose, force water through the pump, then quickly submerge it and it will pump fine. Any ideas?

is your o-ring blown out?

The weight of the boat sitting in the water, and forcing water up to the pump once you open the seacock, SHOULD allow it to prime. The pump probably does not provide enough suction to bring it up from a garden hose in a bucket.

that pump should be mounted below the waterlne since it has no suction capabilities

Exactly! It will not pull water up to prime itself. Gotta be below water line. Even if mounted below water and you have a high spot before the pump, you gotta bleed off that air pocket for the pump to pump. The hoses should always go upwards from the seacock, shut off valve, strainer, and into the pump.

pull the discharge hose off the pump, open seacock, water should flow out...if it does not check for blockage or pump not low enough, also when we install mermaids, we put in a PVC bleed valve to bleed the air out, most boats when pulled out of water and put back in will air lock...some will after running...

One other thing you can do is take out the screws holding the front cover off. remove the front cover and impeller, make sure the magnet and cavity is clean. doesn't take much to bugger the impeller drive magnet