Using bonemeal on an underwater grass block to spawn seagrass doesn't work. It works on any other block, though. I assume it actually tries to spawn normal grass before realizing that it can't because it's underwater.

But it would be reasonable to assume that underwater it does the same it does for any other block, spawn seagrass, especially now that underwater grass blocks are quite a bit more common than in 1.12 due to the lighting changes.