I would feed 100% wet food if I could afford to. If I fed something super cheap I could do wet-only, but I don't want to feed something with poor ingredients.
Right now they eat 1/4 cup of dry food + 3oz of wet food each daily. Most of the calories are from the dry food though.
I used to have a water fountain to make sure my cats drank a lot of water but it broke. I am working on fixing it though.
As far as how much wet food to feed, look on the can for a guideline. You can also check out http://www.petobesityprevention.com
and they have a list of calories in different brands' canned wet food. I wouldn't be worried about obesity with cats who eat only wet food but it will help you figure out how much to feed. An 8 lb. adult would probably need 6oz of wet food a day.