Water kefir grains can be used in coconut water, and to the best of my knowledge, they can exist that way pretty indefinitely. You need milk kefir grains for coconut milk kefir, and milk kefir grains need to spend time in actual dairy (cow or goat) periodically. I make coconut milk kefir for myself, but I also make goat milk kefir for a housemate, and my grains stay alive that way. Other friends of mine have kept their grains in coconut only, and they seem to die/get contaminated within a month or two. I don’t really know much in regards to which is more effective or probiotic-rich.