No you don't need an additional cable.
Ideally you will have the + wire leaving from the first battery to your switch panel and the negative wire leading from the last battery to your switch panel. You would then connect the solar panel the same way.
However lots of boats are wired so that the outgoing positive and negative wires come from the same battery. If yours is wired like this I would connect the solar panel in the same way. At the low charge and discharge currents your boat will experience it does not make much difference.
You will need a fuse close to the battery and should also have a controller (or regulator) between the panels
and the battery