You would use met as well. If you want to avoid using met for both the companion and the means of transport, you can use per for the means of transport (using per is a bit more formal).
- ik reis met Sandra met de boot = ik reis met Sandra per boot = I travel with Sandra by boat
- ik ga (samen) met Willem op de fiets = I go (together) with Willem by bike
Met de is accepted in all alternative accepted translations, Wij reizen met de veerboot is one of those accepted translations. I suspect you made another mistake as well, and for some reason the correct translation shown was using per instead of met de.
- Wij reizen met de veerboot = Wij reizen per veerboot = We travel by ferry
Keep in mind that you never use an article after per. I'd say the best translation of per is by means of it also contains the same more formal connotation, although it's sometimes used in less formal contexts in Dutch as well.