Is this your first? Just a gentle comment, you are not 'having a waterbirth'. You are planning a waterbirth at this point in time. I just say that because things can happen that make a waterbirth a bad idea, or you may simply not like it. If that happens, it can be easier for you emotionally if you have made peace with different possibilities.
9 or 10cm also isn't really a green light to push like crazy. It would be 10cm, and in most cases you should get an urge to push, or just realise your body is doing it (that might not happen if your baby is in an odd position, and is far likely likely to if you have an epidural). It comes and goes in waves, basically with the contractions.
Your MW is also right that simply pushing' like crazy' increases the risk of tears. If all is going well, you just want to push, and probably have some guidance around the time of crowning to take things gently - basically if you take it steady it gives the tissues more time to stretch rather than tear.
Finally, you talk about 'having the baby's head hanging out'. Do you understand how the pushing works? Most of the pushing is when none of the baby is out. It is pushing the baby down the vagina. Then there is crowning, when the head has to be born. That is the bit to take gently. Finally, the shoulders and body are born. Barring complications, the head being out at the body not is normally only a contraction/push or two and is basically the easy bit. If you are going to tear, you probably do it as the head comes, not after.