The bug
The drowned that originate from ocean ruins will place water in their spawn position, even above sea level or underground. This can cause unexpected flowing water or pockets of water underground in ocean ruins.
How to reproduce
Seed: 8518343970203511735
Coordinates: /execute in minecraft:overworld run tp @p 7132 33 7739
Can confirm for 18w22a. Ruins that generate on the surface have drowned mobs with a single water source (which isn't updated).
Example seed: 7536436603878249902
X/Z: 2175 / 3880