Living as I do on the edge of the Amazon Rainforest, I feel rather well qualified to answer this question.
A coconut shell is indeed חותלות - just as we can't eat an orange without peeling it, we can't drink the coconut water or get at the coconut meat without smashing the coconut shell open.
That said, how are you doing that? In my home, a machete is a specialized instrument used only for opening coconuts. In other homes in my area, it's an all-purpose tool, used for gardening, cooking, and home improvement. Which would you in good conscience hold by in your case?
Also, what are you doing with the coconut shell afterwards? Kavana has a role to play in this aspect- if you are keeping it indefinitely to use as a cup,coaster, ashtray, keyholder or whatever that's one thing; if you're just drinking the coconut water and then throwing it in the trash at the end of shabbos, quite another.