Interesting idea.

I can think of a couple of reasons why it may not be practical for small, portable rigs...

First, I don't think suitably cold water can be found just below the surface. You'd probably need water at least 15 to 20 degrees colder than the dew point for efficient collection. Assume a dew point in the mid 70s. That means you'd probably need 55 to 60 deg water. Once you get it to the surface, you need a heat exchanger (think radiator) to circulate a lot of air over (with a fan). I doubt you'd get much water with cool (not cold) pipes without forced circulation of some sort.

But I think I remember reading that for larger installations (land based) this was at least investigated. Don't remember what came of it.