Places where some caves intersect with water biomes such as rivers or oceans will generate a thin wall following the contour of the water, separating the water from the cave, resulting in the water often overhanging the cave separated by only a 1-2 block thick layer of dirt or stone.

Sometimes the cave will generate a 'bubble' in the water biome, surrounded by similar thin walls.

Steps to reproduce:

Generate new world using seed: 4675047371479781160

Go to coordinates in attached screenshots