I've only seen the flash once on the water. BUT a year ago we took a trip to our local observatory for some star gazing. You arrive an hour before sunset and get shown the equipment and get an intro talk during dinner. Then right before sunset we run out with binoculars and stand facing the sunset. The docent said you will all see it. 29 of 30 saw it, he was right.
Here is the setup. We were at 9000' and the horizon was low without obstruction. When the sun is only 10-20% left look thru the binocs. This one set over a pointed peak and I actually saw the flash twice in the same sunset. He explained the 'conditions' are right in this scenario much more than happens on the water. We have done it several times since and when the weather is clear we see it all the time.
It is interesting that I always associated the flash with a marine condition but it is not. <img src="http://www.traveltalkonline.com/forums/images/graemlins/Yikes.gif" alt="" />
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