Hey everyone. I'm a bigtime camera newb, so forgive my butchery of photography terminology. I took photography in high school but remember nothing except that the aperture determines how wide the shutter goes when letting in the light, and the F stop determines how long it stays open.

I'm going on a Caribbean cruise in May and my grandfather sent me his old (we're talking antique, here) Nikonos III underwater camera so I can shoot some pictures when I go snorkeling and scuba diving. It has been serviced, all the parts replaced, all the O Rings fixed, all the bits cleaned and whatnot.

Problem is: I have no frickin clue how to use it effectively. I can replace the film, and flip the switch to take the pictures, but I don't know how to effectively set the F stop, aperture, etc.

I'm fairly certain I've identified the aperture knob and the F stop knob. Correct me if I'm wrong here:

One knob on the side of the lens adjusts the size of a ring of overlapping metal plates under the lens, contracting to make the hole in the middle of them smaller or larger. The settings for it are 2.5, 4, 5.6, 8, 11, 16, and 22 (2.5 producing the largest hole in the ring and 22 producing the smallest.) This is the aperture (I think)

The knob that rests under the button (well, actually its more of a switch) for taking the pictures has settings of R, 500, 200, 125, 60, 30, and B. This seems to determine the length of time that the shutter stays open, 500 being a very short amount of time, and B being until I release the button/switch. This is the F stop (I think)

There's also another knob on the lens opposite from the aperture knob which sets something I'm not certain of. There are two rows of settings that slide around when i turn this knob.
The top row (in red) has settings of 2.75, 3, 4, 5, 7, 10, 15, and 30 while the bottom row (in black) has settings of .8, 1, 1.2, 1.5, 2, 3, 5, and 10. I don't know what this knob does at all.

Can anyone give me some advice on what to set these three knobs to when taking underwater photographs? I understand that these all depend heavily on the available light, distance from the subject, degree of motion, etc. The light in turn depends heavily on a number of factors including water clarity, how clear the sky is, and my current depth. I don't have a light meter (do they even work underwater?) but is there any rough guideline for what the settings should be used at various depths?

I know I'm not giving you guys a lot to go off of, and I'm not expecting miracles, seeing as I'm using an ancient piece of equipment that I'm totally not trained in the use of, but even general tips like "The darker it is, the lower you should set X" would be helpful and very appreciated.