The bug

Seagrass, tall seagrass, and sea pickles won't reduce direct sunlight. This can make seagrass look like it's glowing in 1-2 block deep water.

How to reproduce

Place these blocks in such a way so that the top of them is below air with a sky light level of 15, and stand inside them and press F3 to see that the inside of these blocks have a sky light level of 15. This behavior does not apply to kelp, coral, or any other waterlogged block.