I think it is his responsibility to research it if he is concerned.

You are right in that it is you pushing the baby out, and you need to do what works for you and to be comfortable with your decisions. No need to apologise for stating the truth. If he has fears then he needs to work through them so that he can support you.

If your hospital supports waterbirth, then they should also have resources available to you on waterbirth.

Or Kelly has done alot of the work for you here: Giving Birth In Water - The Benefits of Waterbirth

Personally, water is a great pain reliever, it can give a sense of privacy/security, the buoyancy can help you get in positions for birthing and it can reduce chance of tears.