I'd say the last 5 cm are the most overwhelming, but the first 5 can be the most exhausting, IYKWIM. With both my girls, I had relatively quick, "easy" labours (7 hours). I found that the earlier contractions hurt, but I could breathe through them, and with my second, especially, "manage" them with my mind. But in the later stages, I don't know if they necessarily hurt so much more, just that they come a lot faster, and I think hormonally you start to go a little crazy - with oxytocin and adrenalin, etc. With my first, I panicked at this stage and couldn't breathe. With my second, I was much better (you might be surprised at how much better a second baby can be!) I still felt a little out of control, but no screaming, or panicking. The last stage, from 7-10 (transition) can go in 15 minutes, but it is pretty intense! The only time I remember yelling a bit was when I was pushing, and I was sure I was going to split wide open. (with my second) But in a couple of seconds, her head was out, and I didn't tear at all.
All the best with your waterbirth! I am hoping for the same for myself with this baby.