Bleh the water hm's kinda annoy me
What I think happened is this
2nd gen games came out, they wanted everything improved upon, they wanted more hm's and usable tm's therefore we were given waterfall and whirlpool, maybe because cut takes care of all branch likes objects, and there were already enough rock like obstacles
Then we got to 3rd gen, running out of ideas, but still need to keep every new version fresh, so once again another hm that was waterbased, dive
At least HM's aren't really needed that much in 5th gen bleh