Source? This blue post says, “…there will be one available via Anglers reputation that will be bind-on-account), and it’ll work exactly as before, with the benefit of making all of that character’s mounts walk on water.”
That suggests that the item is BoA but must be used on each character separately to get the effect. In other words, a high level character with Anglers rep can buy it and send it to low level alts.
^^ It’s this.