Howe1243 (or some other combo of numbers) sells coconut shells with moss already on em... on aquabid. (I have no commercial interest with this seller)
Coconuts barely float in saltwater. Do they sink easily in fresh, or do they have do be soaked like wood? It seems like an intriguing idea.
PS: It is doubtful they would decay quickly. They float around in the ocean for months. If they can still sprout after that, it can't rot fast at all. But then, it is the HUSK being exposed to water, not the inner hairy shell.