I think they'll be making a serious public relations and science communication error if they call it anything other than Boaty McBoatface. If the RRS David Attenborough finds a new fish in Antarctica that'll get a minor story on the BBC science page if, on the other hand, RRS Boaty McBoatface finds a new fish in Antarctica then that headline is guaranteed a place in the "Most Read" section of the BBC news site.
Yes, it's a silly name but that doesn't mean it can't help perform a serious scientific purpose.