That's what I always do. I carry my refillable mug with me, and constantly fill it at the water fountains all day long. I've never had a problem doing that, the water is filtered, and tastes just fine to me! I dehydrate easily, so I'd spend enough for another vacation if I bought bottles all day. And I have no desire to shlep 3 or 4 bottles or more to the parks each day from my resort!
Sayhello
ETA: Hey, just thought I'd add that as I re-read my post, I realized it may have come off as rude or criticizing or something. I am in no way trying to imply that people shouldn't get their own water & take it to the parks. I think it's a fabulous idea, but it's something I wouldn't do, because it involves too much shlepping. I also wanted to assure the PP that fountains were available, and could be used as he/she asked!