Caution: Water conducts electricity, making water and electricity a dangerous combination. Outdoor wiring must be designed and installed with great care and in full conformity with local regulations. This will normally require the services of a qualified electrician.
The first design choice is between a low voltage system and a high voltage (mains ) system. Low voltage systems are the most common. They require a transformer and waste a certain amount of power, but the safety risk is reduced and fewer precautions have to be taken. High voltage systems necessitate the use of armoured cable, water-sealed junction boxes and safe cable routes (below digging depth if underground or above head-height if overground).
A RCD (residual currant device) should always be placed between indoor and outdoor electrical circuits, including the initial connection to an outdoor transformer used for a low-voltage system. The RCD can detect electrical faults and disconnect the supply before harm results. The RCD should be located inside the house but plug-in RCDs for outdoor sockets are also available.