OK, I REALLY am going to go out on a limb here and break one of my rules, which is to never reassure ANYONE over the Internet. But I do think that a little perspective here would be helpful. Please promise me that you understand that I'm just talking in general terms here, and not diagnosing or treating you myself, you need to go over the scan with the doctor who ordered it...
The paranasal sinuses are air filled spaces surrounding the nose and draining into the nasal airway. There are four "sets" of sinuses: The maxillary sinuses (the big ones in your cheeks), the ethmoid sinuses (the little ones between your eyes), the frontal sinuses (in your forehead), and the sphenoid sinuses (deep in the center of the skull).
That is, you don't really do anything about it if it doesn't cause any problems, and the only reason that you know that it is there is because you got a CT scan. In some cases, the polyps can be big enough to cause blockage of the sinus drainage pathways, and thus cause symptoms - this often is helped by surgery in which the sinuses are opened up to drain better and the polyps are removed.
Now, your scan mentions sphenoid sinus retention cysts. If you google sphenoid sinusitis (which I am guessing you did), you will find some very scary stuff. That is because the nerve of vision (optic nerve) and the main artery to the brain (internal carotid artery) run through the wall of the sphenoid sinus. For that reason, an ACUTE INFECTION in the sphenoid sinus is something that needs quick treatment, and can have real complications. HOWEVER, a small polyp or cyst is not the same thing as an acute infection (in which the sinus fills with pus). SO, as with most things, "the devil is in the details". An incidental finding of a small polyp in the sphenoid does not always need treatment at all...
It does worry me that you have such severe headaches, but the way that you describe the symptoms is not that typical for sinusitis. It is also unusual that you just go the report on the scan without any context - did it just get mailed to you or something? It's probably better that you speak with the ENT doctor about the findings - maybe you can call the office tomorrow and ask about them...
The bottom line is that CHRONIC sinus disease (with cysts, polyps etc...) is not really an infection at all, but rather a form of inflammation related to poor ventilation of the sinuses. The fact that both an acute infection and chronic inflammation can look similar on a CT (that is "grey") doesn't necessarily mean that they are the same condition (although there is some overlap).
Whether or not chronic sinus disease needs any treatment is based on the extent of the disease on the CT (your scan was read as "mild") and the clinical context (that is, your symptoms). Have your ENT doctor take a look a the scan and then decide with him or her if any treatment is necessary. I wouldn't think that this would prevent you from diving in the future, one way or the other...
Mike