[Phantom]

I have not tried it but I think that the pingpong ball idea could work and if they are to light you could poke a hole in if and fill it a 1/4 of the way with distilled water then seal it with seal-all. That would keep them down they could actually hold some heat but reduce the rate of transfer.

A great way to test this would be to get a few 5gal buckets fill them with equal amounts and temp water. Sit them out in the open just far enough that they will not shade each other and try air only ping pong balls, with water 1/4 filled, with both, and control. Measure the changes in temp over time them immediately retry with new water in the buckets and retest. The reason for redoing it is that the temp of the plastic and water in the balls can effect the water temp until they equalize.