Rubber and neoprene make it very water resistant yet there is breathability, a nice factor when paddling vigorously. Not like a swimmer's cap!
I am a female with long (20 inch) hair. I can either protect just the crown of my head or carefully tuck all my hair inside the cap. The mesh inserts over the ears are a plus, as is the ribbonlike, adjustable chinstrap. Perhaps paddlers who do whitewater or aggressive touring in rocky rivers could wear this under helmets but I have no experience with either.
Have paddled in winds 15 - 35 mph with and without a light rain. This BomberGear piece stayed in place with absolutely no need to stop paddling for adjustments, even leaning into the wind.
It kept my head dry and just warm enough. All this and packs down smaller than a cell phone.
Try the BomberGear skullcap. You might just pack one also.