Sight fishing flats is very interactive. This is one reason I believe it's so popular.
When the sun is out, the bright light helps to create color contrast and cast a shadow from the fish so anglers can see fish movement a little easier. The bright light from the sun also aids in spotting fish at greater distances giving anglers more time to prepare for a possible shot at a fish.
When you have solid cloud cover these advantages are not available. Low light takes away most color contrast and usually there are no shadows to give movement away from far distances.
As an angler you need to adjust for this and not look to far away. Scan the bottom at short distances and prepare yourself for a quick short cast.