how safe are 240V water solenoids

They've been around for a long time. Simpson's were using them in the late '80s in washing machines and dishwashers.

The devices themselves may be intrinsicly safe, but what about the way you wire them up? Are you a sparky?

I realise you want to use the salvaged ones, but have you thought about using low voltage sprinkler system solenoids?

should be quite safe when used as it is inside the washer - ie - used indoors inside an earthed enclosure and (earth the metalwork of the solenoid too - IF its earthed inside the washer).

Also these solenoids seem happy to sit at mains water pressure all day and night for years between uses, and don't seem to leak or drip too.

I remember playing with these about 10 years back and I remember that they seemed pretty safe by construction. If in doubt - use a small isolation transformer or RCD or rewind the solenoid for a safer voltage such as 12, 24 etc. Some have the coil sealed totally though, and this might make things difficult.

Finally - in households with town water - the water itself in the pipes will likely have a good earth connection via underground pipes, domestic water heater etc, but in a situation where there is no earth, and the water is totally electrically isolated (such as water coming via plastic pipes from a plastic water tank) you have to think of safety matters - particularly if its supplying something like a drinking fountain, where the water (if live due to an insulation breakdown in a water solenoid that isnt earthed) makes direct contact with the body, and worse still, the mouth. Also in this situation, you dont want people getting annoying "tingles" from leakage too.