What is a Hazelnut?
A Hazelnut is a diver who enjoys diving in virtually any water. A Hazelnut doesn't need to go on vacation to some tropical wonderland in order to enjoy diving. (The Hazelnut would certainly enjoy the vacation, of course, but they'd be back to enjoying their local water in no time.)
Why "Hazelnut"?
Hazel: A light or yellowish brown color; or a brown, green, yellow-brown, or even partially blue all-mixed-up eye color.
Nut: An enthusiast. (Also, a crazy, eccentric person -- depends on the temperature, perhaps.)