As soon as we got back to the boat, I noticed the problem with the shower valve. It must have dropped below zero for a while at some point, and the expansion had cracked the case. I realised we couldn’t turn the water pump back on until that valve was replaced, so we couldn’t use the sink or basin taps either. That’s why I decided to go straight out in search of a replacement. It only took a minute to get the original off the wall, so I had something to compare against in the shop. I can now report that the replacement valve works perfectly, and is even marginally easier to operate with wet hands.