There's an old schematic of one here. Also wikipedia has two lists (one for ancient and one for medieval) of various watermills, including a list of old texts that include descriptions of them (so you can search for translated copies and verify them personally given the not wholly reliable nature of wikipedia =P). But it gives you a starting point to search for the mills mentioned on those lists on other sites and in books and such.

Most of the listed mills are grain mills but not all of them are.

Also there is a book called Wind, Water, Work, which goes into great detail about mills in the ancient and medieval period.

On a side note it is worth looking up Barbegal Mill. It is a very impressive piece of ancient engineering.