Do you have a Michelin road atlas or the Michelin 1:200,000 scale map of the region? How about the Michelin green guide for the Côte d'Azur? If you don't have them I think you should make a small investment ASAP..
If you look at the Michelin map you will see that the road runs quite a ways inland from the coast most of the way from Marseille to Cassis. The only way to reach the Calanques overland is to hike in. As you suggest you can take a boat trip from Cassis but I don't think the boat trip will let you off and pick you up.
The closest you can get by car to the Calanque En-Vau is the parking of La Maison Forestiere de la Gardiole. From there you have to walk around 3-4k to the calanque.
Here is the map of the area and you road - provence.aparcourir.com/c_geo/geo_loc.php…
You can get to this calanque from Cassis as well passing also by the calanques Port-Miou and Port-Pin, but it is even more difficult road with one quite steep climb.
Everything is well explained there:
It takes less then one hour to walk from the parking to En vau calanque
To get back maybe more... :-)
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