Crab bombs where discovered by generikb where you put a rock on a crab causing them crab the walk into the rock so it breaks into rock shards.
If you put a rock on a crab it will explode (hence the name Crab Bomb) into rock shards which act as shrapnel
If you put a rock shard on a crab then it will create what I will call a "crab cutter", This can be used to break coconuts (not green coconuts) into coconut halves along with rocks into rock shards in a couple seconds, this effect can also be done with normal coconuts.
If places correctly then maybe you can use an unbreakable object to create a small "device" which can be used to saw through coconuts and rocks easily.
Notes- when the coconuts or rocks explode there is a great chance that they will be shot into the ocean (and i say shot because when it explodes it probably goes as fast as a bullet).
If trying to make a crab cutter then use a rock shard instead of a rock or dehusked coconut because it has a great chance of also exploding.
I also suggest for you to use the life raft as a cage so you can easily contain the crab so it doesnt run away and there is a slightly greater chance that you will catch the debris.