So basically what happened was I made a little 1 'block' low, 4x4 block wide, dent in the sand near water, then let the water in by getting rid of the wall of sand stopping it.
Accidentally, whilst moving one of the bits of sand, I picked up the dirt underneath me and went underwater. It has this underwater sound "atmosphere" I guess you could call it? Like if you punched someone underwater it sound more like hitting cardboard than a fruit.
I jumped and placed the dirt back into the whole I was just in, and got out of the water completely. I then proceeded to cut some trees down and noticed that everything still sounded like I was underwater.
I tried everything to get rid of it that I knew of - logging in and out, exiting both my server and game completely and back in, using the "/unstuck" command - nothing would work.
I wasn't sure where else to put this so here we are.