If underwater dirt is in contact with a grass block, it will grow grass at light level 3 or less, and the revert to dirt at light level 4 or higher.

The attached world demonstrates this. The randomtickspeed is set to 500 in this world. It contains a 21 x 21-block (interior measurement) box holding water 2 blocks deep above grass and/or dirt. The front and back of the box are glass blocks to let daylight in. There is a single grass block to seed the dirt to convert into grass. Controls provide for inserting or removing the water, and for inserting or removing sea lanterns at Y=13. The only other control you might need to use is setting the time to noon or midnight.

 Steps to Reproduce:

1. Open the attached world "Underwater grass.mcworld".
2. Observe that the floor of the box consists of dirt near the windows and grass in the middle.
3. Remove the water and observe that the dirt changes to grass as expected.
4. Restore the water.

Expected results:
The grass changes back to dirt.

Observed results:
The grass within 6 blocks of the windows changes to dirt. The grass in the middle is unchanged, even though it's at an even lower light level.

5. Insert the sea lanterns at Y=13.

Expected results:
Either nothing happens (either because the dirt is under water or because the light is too far away to change dirt to grass) or the dirt changes to grass.

Observed results:
The grass changes to dirt, the opposite of what makes sense when you add light.

6. Remove the sea lanterns.
7. Use the command /time set midnight.

Expected results:
Nothing changes.

Observed results:
All the dirt changes to grass, which makes no sense at all.

8. Use the command /time set noon.

Expected results:
The grass reverts to all dirt, since you undid the change that made it turn to grass.

Observed results:
Only the grass within 6 blocks of the windows reverts to dirt, bringing us full circle back to the pattern of the original world as it was uploaded.