In an ocean biome, I saw some seagrass in a cave full of water. The ocean was not connected to it.

Because of the biome and of the water in the cave, I think that the seagrass appears.  

In spectator mode, I saw some seagrass in a cave full of water. I pressed F3 and I saw that the biome was Ocean.

1. Find a ocean biome
2. Find a cave full of water 
3. You are done!