scrying vs hydromancy

What is the difference between water scrying and hydromancy

Hydromancy could refer to just about any divination that uses water, such as dropping a stone in water and watching the ripples or divining from the way a body of water flows, while scrying with water is a more specific practice.

This thread here in the coven has some useful information for you:
http://www.spellsofmagic.com/read_post.html?post=434487

Another link:
http://www.occultopedia.com/h/hydromancy.htm

Blue hit the nail on the head, I believe. Hydromancy is more broad, and would likely involve divination using water in general. There are many different ways to use water in divination. You could note the color, the movement of the water itself, that sort of thing.

Scrying seems to be more narrowly defined. When I've seen discussion on water scrying, it usually involves using a bowl with water and staring into it. The water serves as a reflective surface, not unlike a mirror.

Anyone, feel free to correct me if I'm off!