Summary:
All waterlogged blocks except for hoppers and mob spawners prevent grass blocks from turning into dirt.
The full list of waterloggable blocks can be found on the Minecraft Wiki.
Steps to Reproduce:
Place a grass block underwater
Place a waterloggable block on top of it
Set the random tick speed to a high number
Observed Results:
The grass block does not turn into dirt.
Expected Results:
The grass block should turn into dirt.
This seems to be related to MCPE-135043. Grass converts to dirt under waterlogged blocks at the the same light levels where grass does not convert to dirt under water by itself, and vice versa.