Red Sea is heaaaavy, but thats about it. Their later stuff is kind of what you hear on Wavering Radiant, melodic yet brutal. Oceanic blends them the best, but Wavering Radiant really pushed that style in to a new direction. I personally love WR, but some fans thought it was a disappointment (for whatever reason). Ghost Key is my favorite off WR too, the style of that song is similar to In The Absence of Truth (which was more mellow, but experimental).
If you like ISIS you might like Neurosis (pretty much the founders of this type of music), Rosetta (a little heavier than ISIS) , Pelican (more on the post rock side, all instrumental), and a new band called Mare (they have one EP, but its very good if you like any of these artists I mentioned).
Hope that helps you out a bit.