Grass and mycelium will not suffocate underwater, despite all light levels. I have noticed this problem since 1.13 pre-release 1, and neither of the subsequent pre-releases have fixed it. I reverted to 1.12.2 to see if water really would suffocate mycelium and grass, and it did, especially with a high tick speed, so it is definitely something that has changed in 1.13 snapshots. I do not use Optifine, which I mention since I have seen people claim it is an issue with Optifine, and Optifine doesn't exist for 1.13 yet anyway. Also, mycelium that generates underwater naturally will not revert to dirt, as you can see in my screenshots. The mycelium and grass will not spread when underwater, as fixed in a previous bug, but they will still persist underwater without reverting to dirt.