I have to put 16oz of water in daily, but can't find a leak anywhere.

Possibly leaking into oil through a worn headgasket. If exhaust is a whitish-blue color this is prob. the issue. Check to make sure radiator cap is sealed all the way.

I agree with csubak30 have the cooling system pressure tested if there is no external leaks then it could be a head gasket

sounds like a blowed head gasket