Summary:

Waterfalls can generate sea grass that is not waterlogged. They are not in the water.
It seems to me that the generation of sea grass shouldn't be generated at all in waterfalls (in flowing water).

Examples:

-392114072
-175 64 3411

-392114072
-354 68 3097

Steps to reproduce:

1. Create a world with the specified seed.
2. Visit the specified coordinates.

Observed results:

Sea grass form in non-swampy and flowing water.

Expected results:

The sea grass should be at least waterlogged.

However, it seems to me (logically) that seagrass should not be generated in flowing water (in waterfalls).

Image: