I have seen it just off of the coast of Uahu. The sky has to be clear of couds from you to over the horizon. Wait until just the second the sun sets. Lensing, (prism) from curvature of the ocean causes it. I can't see it from the gulf coast, (curvature of the land to the west blocks it). Key west should be able to see it on sunrise, but not on sunset.
I have never seen the green flash on a sun rise but have seen hundreds on a sunset- not shure why- maybe the dust generated by day helps generate the green flash.