You already answered the question, cubemapping. :)
I've found there is no need to update all 6 cells every update, infact there's no need for a down cell at all, I update 1 cell a frame if I recall, sometimes when i've used it in the past I update 2 or 3 cells a frame, but never all 6 for water because the movement of the water and the lower resolution of the cube map blurs the image enough that a minor imprecision in the reflect isn't noticed and saves a bundle of rendering time.
I've found there is no need to update all 6 cells every update, infact there's no need for a down cell at all, I update 1 cell a frame if I recall, sometimes when i've used it in the past I update 2 or 3 cells a frame, but never all 6 for water because the movement of the water and the lower resolution of the cube map blurs the image enough that a minor imprecision in the reflect isn't noticed and saves a bundle of rendering time.