I have used a reclaimed washing machine valve for watering control for about five years. It's mounted in a box near the outside tap, and yes I was concerned about mains in a wet environment.
My solution is to use two 12V mains transformers, one in the house fed from a timer in the conventional way feeding switched 12V AC to the garden, and the other used in reverse at the valve, ie 12V AC in and 240V AC out to the valve. This means that although the valve is fed with nominally 240V AC it is floating, not referenced to earth, current limited, and all enclosed locally. The feed between the two transformers is safe at 12V AC, and can be on whatever connectors are available without worrying too much about waterproofing.
This arrangement has worked fine, with all bits sourced from the bits and pieces box. If I was starting again from new I would probably use 24V transformers, but otherwise this may be a useful solution with many/most of the parts available free for collectors of electrical "junk".
Charles F