Fillet,
I've touched on this issue myself. I tried experimenting with a baitwell pump doing double-duty as a washdown pump. Obviously, waterpressure was an issue (as in not enough) so I upgraded to a 1000gph bilge pump (always the cheap route!). I hooked up a thru-hull with a seacock and ran a hose from that to the pump. Bad idea as the pump didn't prime very well. So, now I have a nice thru-hull and seacock waiting on me to spend the $100 for a ProBlaster self-priming pump.
I am installing a Y-valve and intend to have the pump pull double duty as described above.
As far as the tank plumbing, I have raw water enter the bottom and old water exit the top of the tank. The baittank fitting you can get for the Blaster calms some of the pressure but it still comes out pretty strong.
Sorry for the long post!