It is possible open a coconut without a tool, but you really need to know what you're doing. ( I can't do it.)
You can Ric, You just need to think about it. You need a green nut to make it easier, and a rock (or a hard place). It doesn't take a lot of thought to get the outside green bit cracked in a few places so the fibres inside are visible.
Once you do that, you can easily peel the green off with your fingers. It is time consuming at first, but after a few nuts you barely think about it.
If you only want a drink, you bash around the end where the stem used to be, and strip as much as possible from there to expose the 3 soft eyes. Otherwise, continue to strip the fibres until you have the nut.
Poke the eyes in with something and drink the water, then break the nut (on a green nut it is relatively soft) and eat the white stuff. It tastes a little weird and fizzyish and rubbery at first, but you soon get used to it.
Contrary to rumour, coconut water is unlikely to give you the runs. I have lived on coconut exclusively for a week at a time without ill effect other than weight loss when I was doing th' wombat experiments in castaway living years ago.
You might remember I sent you timed shots of opening a heap of coconuts and getting the juice, using only a little 2.5 inch blade Swiss Army Knife in 2015 or 2016 when I was living on a tropical island 18 miles off our coast, with no running fresh water for a couple of years.