So I was creating a new world with custom settings and just for fun i slid the "Sea level" bar from 63 to 144. Everything was underwater except for some villages that broke the surface, but i could also see whatever these things were.
It looked like they were made out of bone block, and there was also coal ore randomly placed around the edges. Each had some sort of opening, or way you could get into the structure. And they were everywhere, but randomly placed in the ocean with no rhyme or reason.
I haven't played Minecraft in a while so this could be a dumb question, but what exactly am I looking at here?
Maybe also fun to know: you can easily generate these yourself by using a structure block. Place a structure block, turn it into load mode, then use this name: fossils\fossil_skull_01. There's 1 to 4, and also a _coal variant (example: fossils\fossil_skull_03_coal).
Next there's also a spine variant (fossils\fossil_spine_01) which also has 4 variants and a _coal variant for all four.