I have only been on two liveaboards, but neither put us on a site that required navigation skills. One was mostly drift diving, so the plan was to drift with the current, then get picked up at the other end by the RIB. The other was mostly wall diving, so the plan was to follow along the wall for X minutes, then turn around and follow it back or, in some cases, get picked up by the RIB. I get the impression liveaboards generally do not require great navigational skills, as a LOT of their customers might find that too demanding. In both instances, not being able to make it to the exit point--whether the mothership or an RIB pick-up point--was not the end of the world, as a diver could shoot an SMB and get picked up by the RIB, no questions asked.