i did it a few years ago to Edinburgh (which was a bit longer obviously). the way over was a bit horrible for a couple of reasons - was at 6am, was full of skinhead Rangers fans and there was a problem with the ferry so we all had to sit around on it for an extra hour or something while they figured it out. which also meant we missed an afternoon Fringe show we had tickets for. the journey home was lovely though. always intend to do it again but always end up flying as it’s such a brilliantly short flight, but need to save a bit of money this time round.
apparently you can do a day return for only a tenner. sounds great except that’s like 12 hours of travelling in one day with only a few hours at your destination…