Diving hoods are used to keep the head and neck warm, and this is a very important function since most of the human body warmth is lost through the head. Therefore, maintaining our head covered and not letting warmth leave through it might be more important than it seems.
Like most diving clothing, diving hoods usually come made from neoprene. This material allows hoods to be impermeable and maintain the diver’s head dry as well as warm. Some of them have a zipper in order to make them easier to be worn and taken off. This zipper must be made with an impermeable material and assure the diver that water wont enter through it.
In order to be successful in their function, hoods must seal to the face and neck or otherwise water would enter through those openings and this diving clothing would be pointless. Besides this, some models of hoods are done in such a way that allow the use of other clothes under them in case the diver dives in extreme cold waters and needs extra protection for his head.
If you have a diving hood which is too tight around your face and wish to fix that yourself it is important that you know that this is not easy to do. Actually, it is not recommendable that you try to fix this yourself without consulting someone who knows how to do it properly or otherwise you might end up entirely loosing your hood. For example, it could happen that you fix it in such a way that it does fit your face properly but it ends up not sealing to it correctly and letting water enter through it.
The most important things you should check on the hood you wish to buy are the material it is made from, the way it seals to your face and neck, and the zipper in case it has one. This way, the material it is made from should be impermeable and allow you to wear other clothes under it in case you wish. Secondly, it should seal properly to your face and neck, not too tightly but not allowing water to enter neither; and in case it has a zipper, it should contain a material which doesn’t allow water to filter through it as well.
Source: Dive Pilot