on my hardcore survival island world I was mining when I found a small vertical 2x2 hole filled with water that wasn't connected to the ocean. Idk if its a structure glitch or something else. I'm on singleplayer. Yes I know this sounds like the beginning of a Minecraft creepypasta written by some braindead prepubescent boy. I'm not making this up so please tell me what this is. I am playing on vanilla Minecraft with 1.13.2
1) a cave generated there, and it ran into conflicts while generating with more dominant features such as the 'bedrock' stone of rivers and oceans or with an actual 'water_lake' structure.
2) If you're close to the ocean, it may also be an underwater cave that generated off-shore in an odd shape, possibly combining with case one.
3) Additionally, it may not be an intentional structure at all but just a fluke of hill/slope randomization that made a steep valley, and because it was below sea level (default 63, your world may be customized so YMMV), the master controls of world gen added water to it, similar to how you get shallow lakes and ponds in non-arid biomes whenever the ground is too low.