Normally a cloudy sky makes for great diffusion and solves a lot of the problems that a sunny day creates like dramatic contrast and huge shadows. That may be good for portraits, but not always so good for swimwear. Compare the photo shot on a sunny day to the photo shot on a cloudy one.
The sunny day certainly creates more vibrance, and colors the water nicely. But the contrast overexposes the sunny side of the model and makes her squint. The cloudy day evens out the exposure on the model, but makes for a pretty flat photo otherwise.
Sometimes it’s ok to shoot a swimsuit model in the bright sun – heck that’s what bikinis and summer days and water are all about right?
The solution for minimizing harsh contrast? I think the best solution is to shoot on a sunny day and protect the model from the sun somehow. Either have an assistant hold a scrim or a sun blocking shade to block the model from the sun, but let the sun light up the rest of the scene, or have the model hide under a tree, boat, overhang, or some other shading object.
Be sure to order my DVD for much more on photographing your models in swimwear.