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Thin lycra hood, cool on the boat and protects from sun and most stingers in the water.. Very popular for warm water divers
https://www.makospearguns.com/Camo-Lycra-Hood-p/mclh.htm
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While diving? That is, while on shore or a dive boat, right? Bandana or sun hat for me. Under water the UV should be quite minimal I think.
Between gearing up, waiting to splash, and floating on the surface for pickup can lead to a lot of sun exposure. On a recent Galapagos trip I spent a lot of time every day in the open Panga's, at least a 90 minutes across three dives a day. A light lycra hood (the Mako one in this thread) was really nice for protection.