It is EXTREMELY difficult to transmit HIV through saliva. Possible, yes, but not easy.
This is a very good thing, because if HIV was EASY to transmit, we'd all be dead by now! Consider that every time you sneeze some aerosolized saliva is expelled.....
HIV (and other blood-borne diseases) generally require some kind of access to the blood stream in order to effect transmission. There is also an "amount" of viral material that has to be transmitted for an infection to occur, but the exact concentration/count of viral particles necessary is not known and probably varies widely from person to person.
The kind of "extreme risk" scenario that many activists on this disease like to paint simply isn't true. When you have a disease that has a near-100% fatality rate without treatment, and even with treatment it kills you eventually anyway (just more slowly), if it was EASY to transmit you'd have half the human population dead by now. Remember, we're 20+ years into this disease's "mass infection" at this point.
Don't believe the mass hysteria engendered by the activists over infection profiles in Africa. A little-discussed factoid is that one WIDESPREAD practice in Africa used for birth control is putting Tab A into Slot B-1 instead of Slot B. Why? Well, when you don't have ready access to condoms and pre-packaged forms of birth control, what is the OBVIOUS other choice? (The activists DO NOT like discussion of this, and have successfully quelled the WHO talking about it for what should be obvious reasons. Not talking about things, however, is not the same as making them not happen.)
As for other diseases, yes, you can transmit things by mouth. Unknown to many, an extremely high percentage of people have one of the strains of the herpes simplex virus in their body - the varient causes cold sores, specifically.
Also, the most virulent time for transmitting most "beasties" is BEFORE you have symptoms - you're most likely to transmit the flu or a cold, for example, in the day or two BEFORE you feel like crap. So, while avoiding that kind of "close personal contact" while you're feeling ill is a good idea (you probably won't be diving with the flu anyway!) in terms of efficacy in avoiding transmission its dubious at best.
Salt water is a nice diluent of various beasties, but its not a disinfectant. But since essentially all beasties require some critical concentration to cause an actual infection, it may be good enough.
Bottom line? There is risk in scuba diving. There is also risk riding in the subway with someone who happens to have active TB next to you who coughs. Deal - its called life, and none of us get out of it alive.
