The journey south to Massilia is in a pair of small transport vessels.
Transport craft fall into three classes. The smallest are flat bottomed open topped craft that can be dragged across shallows and beaches (portages) by one or two men. Able to bypass unnavigable portions that way, these craft are used on all rivers because they are very flexible and convenient. They will easily founder in open seas, though.
The intermediate vessels cannot handle the smaller rivers, but can manage coastal water.
The largest are sea-worthy and seen from a cost per ton of cargo basis, the most efficient. But they can only enter the largest rivers.
The party proceeds south in two "intermediate" ships. There is only a small cargo space below the single planked over deck, and it's loaded with goods belonging to House Palantus and Einar. These vessels normally do not travel continuously, especially on rivers, where they require dayling and eyesight not to ground on a shallows. They have to stop along the banks and make camp at night. It is a two week journey to Massilia, most of it through the Rhone valley, which is occupied by the Visigoths. But House Palantus is still in good standing with them, and Einar and Titus have experience: as long as the fees (and bribes) are paid, there should be no trouble.
As long as they don't see pointed ears. That can set them off.
"Intermediate" would be the smaller of these two vessels.