I’m assuming that Braavos (the City) was attacked sometime during the Century of Blood. Everyone keeps going on about Braavos’ excellent defenses, with her navy and the Titan (sea-wall islands included) making her near indestructible, yet she has one obvious, potentially crippling weakness. The Sweetwater River, the aqueduct feeding the city, if taken out could cripple Braavos for years. Is this a viable option to combat Braavos’ seemingly unbreakable reputation?

I would say that it’s a good rule of thumb that, given Braavos’ survival and indeed dominance of a continental political world that is near-constantly at war, someone in Braavos has probably thought about this rather obvious weakness and has done something about it.

I would imagine the Braavosi Coastland absolutely includes the source of the Sweetwater River, and that they have well fortified that, keep a close watch on the Norvoshi to make sure that they don’t make a push, and bribe the Norvoshi government regularly to make sure that anyone else who tries, dies.