Stephan,
I love the Water Puppy pumps and have used them for everything from shower sump pumps to bilge pumps, and it will be perfect for people who only use the original Amel installed outlet for the chain wash. I think for your application, where you have a 3-way valve and hose for targeted spray, you might find something else better suited.
As good as the Water Puppies are, they are relatively low pressure, and will not work well pushing water through a spray nozzle on a hose. And if that nozzle can shut off the water flow, the pump will have problems when deadheaded.
The problem with many of the other pressure pumps sold for wash down service is that they are diaphragm pumps. When used below the waterline a tear or other failure of the diaphragm means lots of water can flood in very quickly. That's one of the reasons you have found so many do not recommend installation below the waterline.
You might consider one of these of a size you consider suitable:
The only possible leak point is the shaft seal, which--even if it was to fail totally--will not allow water to flood in quickly. They are self-priming, so if you wanted to move it to a level above the waterline it would be fairly easy to find a suitable spot because they are very small in foot print.
I have been using this style pump as my freshwater pressure pump for a year now without problems. That's probably more hours of operation than you'd ever accumulate in anchor wash down service!
Bill Kinney
SM160, Harmonie
Boston, Mass