If you leave a grass/mycelium block under water, or if there is a neighbouring grass/mycelium block near the water, it will spread to water blocks occupied by kelp, corals, coral fans, sea grass, sea pickles, and any waterlogged blocks. Kelp pops out after that, other blocks remain. See screenshots.

P. S. Also, grass and mycelium do not suffocate under water, both normal water and waterlogged blocks.