Pressurized water system has a switch in the galley. Horns corrode and fail.
I would start with the fuse block. Theoretically, the fuses with no wires are spares. Those which do nothing AND have wires connected are not functioning. A good place to start is to verify whether the fuse is blown or not. IF not, try unseating it and reseating it. That scrapes the corrosion off of the fuse blades and the fuse bus and can often fix mysterious issues.
If non of that works, then I would use a multi-meter on the connections on the device (e.g. pump, horn, etc) then start working backwards to the fuse panel and the dash switches.