"When were you on that boat?"
Translation:¿Cuándo estuviste en ese barco?
There is no need for "tú" because "esuviste" is already in the "tú" form.
You could however add it to emphasise that you are specifically asking about "you" (as opposed to someone else).
Because this is a question the "tú" would have to come after the verb...
- ¿Cuándo estuviste tú en ese barco?