There are these contraptions that condense water from the air, though they are very slow since there's generally not that much humidity, and the contraptions don't have much temperature gradient available to condense with.

But suppose you're near the ocean, maybe even on a boat. Won't there be tons of water vapor near the ocean surface from sunlight hitting the water? And you've got lots of cool temperature available a few feet below the surface.

So would it be practical to collect fresh water by lowering some kind of condensing apparatus into the water, and using it to condense water from the air above the ocean? Could this be more efficient than reverse osmosis?

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.

What I really need is a droid that understands the binary language of moisture vaporators.

Yeah, I wasn't thinking of something pocket sized, but more like something that could be installed on a boat, or on a stationary platform near coastline. How deep would you have to go to get the required temperatures? I was guessing something like 30 feet might be enough.

Dunno. I've never dove the tropics. Surface temp right now is in the 80s, though. I looked for vertical temperature profile data before responding, but couldn't find much. Any scuba divers out there with tropical experience?

It sounds like it would be easier to distill the water with a fire. You'd need a more or less sealed pan to boil water in, some tubing that would pipe the steam into a collecting bowl, and something to cool the tubing and steam down - eg another bowl of cold water. And a fire. Not a lot of gear, most of which would have alternate uses. I am sure I've seen it done on a Ray Mears programme.

Trouble with that is it needs a lot of fuel. I'm asking this more related to interest in making water over long periods, rather than in survival situations (that's why I posted here in the campfire section instead of the survival section).

I still think there's maybe something to the idea.

Yes, it's not so much that it wouldn't work as that other approaches would often work better. If fuel is scarce than maybe a solar still.

Maybe something like, 2 transparent plastic bottles with a pipe between them. Put a little sea water in one and hope some if it condenses in the other. You could speed up condensation by putting the empty bottle in the shade and in cool water, eg hanging over the side of the boat. You could speed up evaporation by letting the sea-water bottle get hot, eg by putting it in the sun and on something black.Cloudy water might do better than clear.

There is. Google for "OTEC". Stands for "Ocean Thermal Energy Conversion". I found several sites that talk about extracting energy from the temperature difference in the water column. One of the side effects is the production of fresh water, both in the way you described, and in more complicated set ups.