Here's my take on all of this:
First, let's get our nomenclature straight. Most extension tubes are 1:3, 1:2, or 1:1. The second number refers to the size on the image on the film as it relates to the actual size of the subject. For example, a photo taken with a 1:2 tube will produce an image on the film that is 1/2 the actual life size of the subject. Another way to think of it is that a 1:3 will frame a subject about 3 inches high, a 1:2 about 2 inches high, etc. To produce a 2:1 image (twice life size on the film), you must stack tubes.
I recommend to Jim that you upgrade to at least a Nik III. If you continue to use the II, make sure that your processing lab does not cut your film. The Nik II does not frame evenly (especially if you don't cock the shutter perfectly each time), and the lab is very likely to cut your images to pieces if they cut your film. You can get a Nik III on Ebay in the $200-300 range. Great camera and you can use any of the modern strobes.
Check out Jim Church's "secret exposure tables" for using extension tubes with manual rigs in his excellent book Essential Guide to Nikonos Systems. Jim allowed me to reproduce the tables in the appendix of my book, The Certified Diver's Handbook (no plug intended, but it's the only other place I know to get the tables).
Extension tubes corrupt the f-stop setting on your lens. An f-stop is simply the result of dividing the diameter of the aperture into the focal length of the lens, which is the distance of the focal center of the lens when focused at infinity from the film plane (which is why f8 on one lens will allow exactly the same amount of light to reach the film plane as f8 on any other lens assuming the same shutter speed and/or strobe light). Since the tube moves the focal center of the lens away from the film plane, a setting of f22 on the lens is actually more like f45 with a 1:2 tube.
Lens focus has very little effect on tubes. I always focus my lens at infinity with a tube. This alleviates a possible mistake when I switch to the close-up kit, which requires focus at infinity. I focus my lens at infinity on a tube even though the manufacturer recommends focusing as close as possible. The images are fine, and I feel that the infinity focus allows me to frame without touching anything. However, it's a fine distinction because, as I said, lens focus has very little effect.
Be very careful with your buoyancy when using a tube and framer. Not only must you be careful not to damage the reef, but the framer is a lever that can easily break the seal between the tube and the camera, resulting in a flood.
All framers are pretty much the same, at least lengthwise. I like the Sea & Sea framers because they're solid plastic and won't bend (a bent framer is ruined). Also, the S&S framers have threaded vertical posts that are easily removed. Many framer shooters cut at least one vertical post, sometimes both, to avoid shadows if the strobe is not perfectly centered or to make the rig small enough to get into small places. -Clay