The hotel gym. You know, that place no one uses.
Come on, it's been a while since that's been used. If anything, it's more like hitting the restaurant marathon. But for staying hydrated, One Mile at a Time recently pointed out that the gym may be the best source for free, bottled water. There's nothing like traveling to put a massive strain on the body.
In lieu of cases where water bottles aren't available, many hotel gyms may also have a water cooler where guests can fill up.
(It's worth pointing out that we also almost always travel with our own Thermos. Handy for spill-proof coffee, water, cocktails on the go. Not that we would know.)
Travelers that have hotel status may also have some perks that enable them to free water. For the full list, check out the blog which gives a full rundown (or drink straight from the tap).