There were always two potential ports in Aquitaine. Bordeaux itself and Arcachon, or at least somewhere in the Bay of Arcachon.
Even in Roman times there had been plans for a canal to link the bay to the Garonne.
There is certainly was potential for a canal as the feeder for the bay comes very close to a tributary of the Garonne – at a village called Paris!
Plans for the canal were always abandoned not because there was no viable route but because the entrance to the bay of Arcachon was guarded by shifting sands and always considered too dangerous to be a major port.