Let's say there's a lot of rain and a pond which is a lateral hazard overflows onto the fairway by a yard or so. If a ball ends up in the overflow but is still outside of the red stacks, how would you treat this?

The water outside of the red stakes is considered "casual water."

EDIT: Here are the pertinent excerpts from The Rules of Golf in "Section II Definitions."

Casual Water

“Casual water” is any temporary accumulation of water on the course that is not in a water hazard and is visible before or after the player takes his stance.

Water Hazard

A “water hazard” is any sea, lake, pond, river, ditch, surface drainage ditch or other open water course (whether or not containing water) and anything of a similar nature on the course. All ground and water within the margin of a water hazard are part of the water hazard. When the margin of a water hazard is defined by stakes, the stakes are inside the water hazard, and the margin of the hazard is defined by the nearest outside points of the stakes at ground level. When both stakes and lines are used to indicate a water hazard, the stakes identify the hazard and the lines define the hazard margin. When the margin of a water hazard is defined by a line on the ground, the line itself is in the water hazard. The margin of a water hazard extends vertically upwards and downwards. A ball is in a water hazard when it lies in or any part of it touches the water hazard.