I know there are many who swear by slugging down liters and liters of water every day, but I'm not one of them. I've had too many docs wave their hands at me and say the whole 64 oz per day fad was a huge mistake by the media - that the body needs that much fluid, yes, but readily gets it from food on top of liquids. All sucking down big water bottles all day does is overwork your kidneys and use up trees for toilet paper. That said, some of us do walk around mildly dehydrated, unknowingly. Something in the middle, perhaps? I have also read, definitively, that coffee and tea DO count as liquids and that coffee's diuretic effect is irrelevant.
I'm really sorry you can't get dasatinib. When I was on imatinib, I had quite noticeable improvement when I went down from 400 to 300 or even 200. Back then, however, they didn't let you stay there, even if your PCR was going down. Today, if you are heading into Deep Molecular Response (<0.01) then maybe your doc could drop you down to 200? Good luck to you.