Not sure how long this rule has been around, but comes to mind each spring: The 100 degree rule.
Found a reference online, as related to water skiing:
"A good rule of thumb to follow for safe waterskiing is the "100 degree rule." This means the air temperature plus the water temperature should be greater than or equal to 100 degrees F to be skied comfortably. You can certainly ski in temperatures lower than the combined 100, however, most people feel that it becomes uncomfortable at that point."
Of course while kiting or windsurfing it isn't so easy as having the boat pick you up if your wake board breaks, so other considerations to stay safe in cold water.
It was pretty quick to add this water temperature visualization, since I already had most of the data available. Right now it only includes the west coast. Looking forward to the water temp hitting 50 degrees in the Columbia, as that is usually around the time I don't need gloves anymore (depending on air temperature).
A quick google after the fact found this:
Not quite as pretty (and doesn't include the Columbia temps), but gets the job done.
Here is an enhancement of this data:
Another random water temperature visualization: