I suspect the manufacturing of riptide signs is a niche market, but when they are used, they are really important! Their placement around the water is such to catch the eyes of those approaching, just as we place objects of importance on the intersection of a third in our photos.
The interesting thing about the rule of thirds in an image is that it can be applied not only to the placement of the intended subject of a photo, but also to the middle ground and background components of an image.
The sign is placed on the left third of the frame, dividing the vertical. It also helps to divide the horizontal space. The top of the sign and the rocky reach create the horizontal thirds.
Also, the horizon in this image is in the middle, but the bottom half of the frame uses the rule of thirds, two-thirds rock and one-third water. This helps to soften the actual horizon line.
As you compose today's photo, look for less obvious ways to apply the Rule of Thirds.