They do need to be primed. In many cases, the pump is low enough that displacement will flood the intake hose up to the pump when the seacock is open. In my 97 2800, that was not the case as the pump was placed above the water line.
I installed a Perko inlet connector just before the seawater strainer. It allowed me to connect a garden hose for priming, flushing and winterizing. i've opened the top of the seawater strainer and primed from there as well.