Minecraft wiki entry for Seagrass, associated text: Using bone meal on a non-transparent block‌[Java Edition only], or dirt, coarse dirt, sand, red sand, gravel, or clay‌[Bedrock Edition only] that is underwater generates seagrass on that block and its surrounding blocks. In order for this to work, there must be 2 water blocks above the block the bone meal is being used on, and the lower one must be non-flowing water.