Somewhere downstream of the Mud Gate
Sorry, can't seem to give you anymore information than that.
As you can see in this video, Bronn is standing on what seems to be a rock bank down stream of the Mud Gate, close enough that he can see Tyrion's fire signal, and far enough out that he could hit the wildfire soaked water near the bulk of the Baratheon fleet.
Tyrion and Joffrey's party are seen standing atop the battlements, near a gate, that gate was predicted by Tyrion to be the focal point of the Baratheon fleet's attacking point during the Battle of the Blackwater. A clever
half-man who predicted this correctly.
Whether the wildfire laden ship approaces from the Blackwater Rush or merely in the Bay is irrelevant, as not enough detail is shown about this area or scene. Most likely though, it would have approached from the Rush, or even simply from the gate.
An addendum about the books
As pointed out in the comments by TLP, Bronn has nothing to do with the lighting of the wildfire, it is in fact a burning ship that plows into the wildfire laden one which triggers the explosion.
Stannis' fleet is allowed to come into the Bay, lured in fact by the entirety of Joffrey's fleet. They are cut off from escape by a gigantic chain, so they are well into the Bay.