I would think it goes without saying one would drain it first, or drink from a straw directly through an eye. One could take a clean slice right at the eyes' junction to sip, pour, or straw, but I thought the question was about opening coconuts. Alas, that is a vague question, asking for a kinfe to "open a coconut" as that can be interpreted a couple ways, but yet still, even after drinking the water, the coconut need be "opened" (read : split) to harvest all the jelly/young meat. Since a young coconut is consumed in entirety & not harvested solely for its water, I assumed "open" meant split. I mean, any monkey can sink a straw in a coconut eye so I reckoned he wants to eat the meat, and doesnt know what knife to use to open it.
Personally, I have scores of coconut palms on my property/neighborhood, and as a native Floridian have been cleaning coconuts since I was a child. No store bought formaldehyde peeled nuts here. I harvest a bunch, and just clean the husks off with with a machete. YOUNG COCONUT MEAT, RIGHT OUT THE SHELL, KISSED WITH SRIRACHA... YUM, MAGNIFIQUE! OR WITH LIME & SALT... AREBA AREBA! Simple pleasures of living in paradise.