Couldn't there be some way to encourage the Casse AI to cross over, they never seem to build any fleets, perhaps adding an already build shipwright to the rebel city next to them (as they don't seem to be able to build shipwrights in Camulosadae). The pirates are also very strong in the north waters and clearly discourage them.
Or make sure the 2 Belgae settlements rebel to them if they rebel that is, and/or make the Belgae more prone to rebellions. They just seem to be kept out of the action.