Green flash is a misnomer. It’s more of a green “wink” -- hard to see unless you’re watching closely. Blink at the wrong moment and you’ll miss it. For a really good sense of how it forms look at the sun through binoculars -- BUT NOT UNTIL the sun is 80-90% below the horizon. What you see is a green edge form first at the sides of the sun and then work it’s way across the bottom and then across the top edge. The middle of the sun (what remains of it) will be orange/yellow surrounded by the flickering green edge. As the sun dips below the horizon the green edges collapse on themselves and in the last instant it “winks” all green.
I always thought it was an urban, no maritime myth -- but it’s real and has nothing to do with the rum you may be drinking at the time.