From GoToCayman.com - the Wiki for Cayman
Cayman runs the same electricity system as the US/Canada - 110/115v with 2-pin plugs.
Most of the water supply comes from the de-salination plant and is perfectly safe to drink. In some more remote areas of the island, well water is sometimes used for the toilets and washing - it apparently tastes quite brackish but makes a lovely cup of tea when boiled! Bottled water (both imported and locally produced) is widely available.
Cayman uses US-style telephone sockets.