Vamos en Coche

OK, to see the distance of the car rental supplier to the Ferry Terminal do a search WITHOUT ticking the box "Return to a different location" and search "Algeciras Ferry" and choose "Ferry Algeciras - Muelle Galera" from the drop down menu together with dates and times and press SEARCH.

Next to each car photo you see a little map. Above the little map it says how far the supplier is from the location or address you searched for. Click on the little map to expand, you will see the exact address and the route to the terminal of this specific supplier. Also, the page is set to "Sort by Price", follow the next instructions to "Sort by Distance":-

During the booking process once you have entered your search criteria, you will see car photos (results). At the top right hand side is a drop down filter, choose "SORT BY DISTANCE”. This will shuffle the available cars based upon nearest, with a small map. The specified distance above the map is the distance from the inserted desired location to the location of the nearest supplier. Click on the little map to expand and the exact address/location will be shown.

No, rental car suppliers do not provide shuttle bus service that near to the ferry terminal. But I would choose Enterprise in your case. Enterprise is the nearest to the terminal: 300mtrs .