Candlekeep can only be accessed via the front gate.
From the Sword Coast Adventurer’s Guide:
The only gate into Candlekeep stands at the end of the Way of the Lion, which is the only road that provides access to and from the outside world. The route extends from Beregost, leagues away, and winds a lonely path out on the peninsula where Candlekeep stands.
This image from Baldur’s Gate: Descent into Avernus gives a better idea of the local topography and the impracticality of a dock:
The nearest proper port is probably the harbor at Baldur’s Gate. See here:
The SCAG goes on to describe the exclusivity and security of Candlekeep. If there is a back entrance, it is almost certainly a well guarded secret.