This report is a duplicate of: MC-214783

The bug at least from my perspective appears to just stop the seafloor from generating beneath y=48. I attached some images to show that this is a real thing you're welcome to recreate it on your own. the best way to describe it to someone who doesn't know the code from someone who thinks they do, would be like this. Do you know how grass blocks generate on the surface? well that's (more or less) the case with gravel on the seabed, its a 1 block layer of gravel. I have no clue why it's happening but it is, also I could be wrong and it could be y=47 if that helps at all. 

PS. loving the update
-snowfish