The bug
Found some kelp that spawned on the trapdoor on a sunken ship. The issue is when you open the trapdoor it stays floating in the water above it.
How to reproduce
Seed: -8300275442758179407
Coordinates: /tp -407 50 2254 (for stairs and trapdoors)
Coordinates: /tp 514 56 1536 (for chest and slabs)
Checked the same ship wreck and a bunch of others, but it looks like the kelp issue is fixed in 18w16a.