My computer isnt the fastest when it comes to loading new chunks when I'm boating on an ocean. And when you are in an unloaded chunk some weird things start happening. I believe it just had some trouble placing me in the right spot after it loaded in the chunk.

 How it happened: I was boating on an ocean (obviously) and there was an unloaded chunk that I rowed into. When my computer loaded this chunk it put me and my boat 12 blocks under the surface. I also could still breathe. Also while 12 blocks under if I stop moving my boat will shake (for a lack of a better term) up and down about one or two block pixels. Also, no mobs would attack me while I was in this state.

            When I got out of the boat the boat just stayed there idle and was stuck. (I didn't try getting back in, there was a drowned throwing tridents at me after I got out.