As you probably know, you can loose a lot of heat through your head and cold water can make that more quickly than cold air. 'Ice cream head' is a realy danger when swimming in cold water and you should choose your headgear with the same care as your other clothing to help keep warm.
Our neoprene hoods are made just like wetsuits. They trap a layer of water close to your head which can be warmed up by it. If the hood fits well then the warmer water will stay in there and your head will be warmer than without.
Hoods are popular with divers where they are underwater all the time. Winter surfers also like hoods and the protection they bring. Some swimmers find a hood is too cumbersome for them and find that a neoprene hat or beanie is a better choice.
Sea kayakers rarely wear helmets so will require some warmth layer in colder conditions. Preferences vary with choices including neoprene beanies windproof outdoors hats as used in hill walking etc.