The cooling system on my sailboat diesel engine has raw seawater
passing through a heat exchanger to cool the coolant. Here is the
flow path.
Seacock-hose-raw water strainer-hose-water pump - hose-heat exchanger

For some reason it does not want to pump and is acting like a pump
losing prime. The hose from the strainer to pump is clear so I can
see the water beginning to flow but it will not make it to the pump.
I have verified that there is no blockage because i can blow through
from the water pump outlet hose all the way through.
When I put the clear hose going to the water pump directly in a
container of water, I get flow and suction and pumping after I prime
it. When I go through the strainer, I get less suction and less
pumpingafter priming but this priming is more difficult. Priming from
the seacock is very difficult and I let in a lot of air when I do but
this really should not matter, eventually it should suck up water.
But it doesnt.

Pumping from the seacock, I am surprised at the amount of air going
through the system.

It seems to me that this thing should pump even if it does lose prime
because you would expect it to lose prime as the boat heels and waves
pass under. Any thoughts?