AC Transit bus line 51B runs between Rockridge BART, through downtown Berkeley, and along University Ave as far as the Berkeley Marina. The runs that extend to the Marina are less frequent than the regular runs (which end at the Amtrak station at 3rd St/University Ave), but still, there is a way to use public transit between the Marina and other frequently-visited parts of Berkeley. Line 51B - University - College - Rockridge | Alameda-Contra Costa Transit District (actransit.org)

Also, there is/was a small ferryboat service that ran a few times a day (weekdays only) between the Berkeley Marina and the Embarcadero in San Francisco. It's been suspended indefinitely because of COVID, but hopes to resume service eventually. Berkeley - SF Commute - Tideline Marine Group (tidelinetickets.com)