Okay, so I was swimming to a destination with my follower, Ralis, in tow. He gets stuck somehow between rocks so I continue without him, thinking he'd eventually catch up, but he's gone!

This has happened before, where my follower just vanishes, and then after a week or so (in real time) he came back. This hasn't happened with Ralis. If they get stuck and then aren't with you when you next load the game, is it worth going back to where you lost them or try and go back to where they originally came from (and if so, where exactly in Solstheim would I find Ralis?)?

Have you tried to fast travel somewhere? This usually will make your follower turn up again if they are stuck somewhere.