6.2 merc horizon running on hose question?
 

My brother is in the process of buying a formula 280 ss with twin 6.2 horizons with bravo3's.

it has sea strainers and water pickups in the hull. I thought i could just take the covers off the strainers, shut of the pickups and run it on the hose. When i tried this on both motors , the sea strainers stayed full and didnt suck up any water. Am i missing something here? do i also need to put the muffs on the drive.
Compared to my bravo ones on my 5.7 mpi's this is greek to me. any body know what to do?