I have seen several scientific studies reporting that radiative heat loss can account for ~50% of human heat loss (I've come across reports as high as ~60%). I do not have references at hand, unfortunately. But the results seemed widely supported.
In particular response to OP, I'm guessing most pool floats aren't particularly light... FWIW, when I got too cold on my NeoAir this fall, I wrapped myself in an emergency blanket ala VBL and was then fine. Controlled evaporative and radiative heat loss. If you completely wrapped the float in an emergency blanket, it would be warmer. After all, that's the basic premise of the insulating ability of the NeoAir...