You can charge it with the energy of water. If you want to infuse other energies in the water, you can leave it in the sun for a day, out overnight, let a plant or mineral steep in the water, and so on. I wouldn't ingest plant or mineral water for safety reasons (as some are water soluble and can cause health issues if poisionous or in too great of quantities). I wouldn't put the necklace in the water for the same reason, though you don't really need the water physically there (this helps some practitioners and doesn't for others - it is a personal decision).
The simplest way is to recognize your intent as charging the necklace with the energy of water.