Micarta, G10 and Formica are all essentially the same thing. As told above, G10 is formed with fiberglass cloth, while Micarta is made with canvas, linen, paper, or other similar things. Formica is the original discovery, and all three are equally durable.
It is incorrect to say that micarta absorbs water - when properly made, the epoxy-like substance thoroughly saturates the fabric (be it canvas or fiberglass), and nothing further can penetrate the fabric. However, I've noticed that the surface fibers on some of my micarta handles can absorb a small amount of water (I have to assume that the fibers have been loosened in the grinding and polishing) and swell slightly, giving a slightly rougher texture to the handle.
If you want a durable yet lightweight knife handle, look for one made from bamboo. I have a Bark River knife with a bamboo handle, and my son-in-law has several, and they are both very light and very tough. They are great for backpacking!