Never had any direct experience with this kind of issue, but I have some ideas.

MiO water flavoring is ok. It comes in all sorts of flavors-- even a tea flavor (as the caffeine in real tea can dehydrate you.) Crystal light packets, too.

Do you *have* to drink just water? I would guess that you'd hate boost and ensure by now - but of course those are always good options. Juices of any flavor. Grape juice bottles are always given out when my husband gets his treatments. He doesn't want to eat or drink very many things, but he sure does like his welches grape juice now.