Referring back to a previous post, I have indeed found the blockage in our raw water system. It was right where I thought it would be.
So I took the plate to the shop and pickled it in concentrated hydrochloric acid to get rid of the remaining lime. And guess what I found? The spud where the hose attaches was cracking loose from the attaching brazing (this is an iron casting, and it had been repaired already by brazing on a new spud). And inspecting the spud itself, I found that it was paper thin. So OK, time to replace it.
It was the work of just a couple of minutes to break off the old spud (that is frightening in its own right). Then grind the old brazing down and braze on a new piece of 1" copper pipe (the old spud was steel).
When knocking off the slag from the brazing exercise (using the correct tool: the handle of a pair of pliers...), I actually knocked a hole in the plate! It's visible there in the photo. Here's a better look from the inside:
And there is a crack there too. Yup... your conclusion is the same as mine: there isn't enough good metal left on this thing to put it back into service.
So, here's a case where a benign failure at the dock was a disappointment, but it was far better thing than if that end plate casting had failed out on the water somewhere.... and surely it would have.
Postscript: I checked - I'd have to order a replacement out of England. Instead, I have a local boat yard (Miller & Miller - I really cannot recommend them highly enough) fabricating me a new one out of mild steel plate.