The standard bolts holding the face plate on the pump are made of brass and mine were obviously overtightened.
We were due for an autumn lift out and bottom scrub so the engine was running a bit hard.
Two of the bolts that sheared were adjacent to each other so the seal failed and pumped sea water into the engine bay.
The engine was ok because it was stilled being cooled ok but a lot of lovely cooling sea water was not going over the side!
I replaced the screws with stainless ones with a bit of loctite and all is ok.