Island-hopping in the Caribbean. . . exploring the Florida Keys. . . diving off Baja, California. It's the experience of a lifetime, especially with a camera.

Even without scuba diving, you can go home with spectacular underwater scenes on film, if you can snorkel or hold your breath for 30 seconds. And you don't need an expensive underwater camera. An underwater housing on the old camera will do the trick.

You'll find, however, that using filters and flash, focusing and getting the right exposure under water will be different from what you're used to on dry land.

Since water absorbs red light in the spectrum, you need to use a color-compensating filter to rid your shots of an annoying bluish cast -- especially when shooting on overcoat days or in deep or murky water. Use a CC30M filter to reduce the blue cast in fresh-water lakes and turbulent seas. In the Bahamas and other clear-water areas, a CC10M filter will probably be sufficient.

Unless your metering system is the through-the-lens type, follow the filter manufacturer's suggestions for increasing exposure according to the filter factor.

You can eliminate filters with an amphibious flash unit (or a standard flash fitted with an underwater housing). Flash balances underwater colors. Avoid using flash in murky water, though. When plankton abounds, firing a flash scatters the rays, bounces them off the suspended plankton particles and fogs over the shot. It's like shooting flash above the water in a thick fog. The results won't be very impressive.

If you must use flash in murky water, mount the flash unit on an extension arm above the camera to minimize backscatter from particles in the water.

Focusing underwater can present problems. The refraction of light rays through water magnifies everything slightly, so objects appear to be only three-quarters of their actual distance away. As a result, you must get farther away from your subject to fit it all into the scene. For this reason, the most effective focusing technique is estimating focusing.

In order to compensate for errors in estimated focusing -- and to get sharper photographs in general -- use a narrow aperture whenever possible -- f/8, f/11 or f/16 -- for the greatest depth-of-field, or area of acceptable sharpness. A wide-angle lens on interchangeable-lens cameras will produce greater depth-of-field, too.

A wide-angle lens also will allow you to get close to your subjects (a boom in murky water) and still get plenty of the surrounding scene into the viewfinder.

Some photographers find metering bothersome underwater. Most housings allow for control of such devices as meters, shutter-speed dials and so forth. But seeing the meter needle in the finder while looking through mask, housing and finder-window may be tricky..

To side-step metering, a good rule-of-thumb is to open up the camera's aperture 1/3-stop beyond the above-water reading for every 3' you dive in murkyy water -- about 1/3-stop for every 6' in clearer water. Bracket important shots: take several of the same shot at different settings.

The easiest way to get perfectly exposed shots underwater is to use an automatic-exposure camera like a Yashica FRI, Nikon FE or Canon AE-1. Set on automatic, these cameras compensate for darker surroundings at deeper depths, as well as for any filters.

There are other uses for a camera with an underwater housing. For fans of whitewater canoeing or rafting, housing protects the camera while affording some fantastic low-angle shots. Ski photos and beach scenes are easier on the gear when the camera is waterproofed.

The limits are bounded only by your own creative juices. And some pictures can be close to home: a dozen sets of toes straddling the bottom of the clubhouse pool or a bathtub full of bubbles shot from the bottom up. KEEPING THE CAMERA DRY

For 35mm SLRs. Housings are available for most popular brands and models of SLRs, including Nikon, Canon, Yashica, Minolta, etc. The majority of these housings list for around $200. But more complex or professional housings may run as high as $600 to $700.

For 35mm Rangefinders. Housings for such RFs as Konica, Yashica and Minolta are widely available and generally list for $50 to $75.

For 110 Cartridge. Housings for such cameras as the Vivitar and Kodak 110s are available for the bargain-basement price of around $40. With most housings come a pistol-grip handle and a flash-cube outlet.

For Movie Cameras. With the advent of XL (low-light) movie film and equipment, amateur underwater movie photography hasbecome increasingly popular. Housings for such cameras as Kodak, Fujica and Bell & Howell usually begin around $100.

For Flash. Chances are good you can use your flash system underwater.

If you flash bulbs or clubs, spraying the terminals with a lightweight oil before diving should provide protection. Most bulbs can stand pressures down to about 200 feet before imploding.

If you have an electronic flash unit, there's probably a housing to fit it for $100 or less.

Specially built seagoing strobes or amphibious strobes range from $200 to $800. Some of the manufacturers currently marketing them are Oceanic Products, Sub-Sea Products, Sunpak and Nikon.

For the Hard-To-Fit. Finally, if you have a hard-to-fit camera (the Xenon Super Shot IV?), don't give up hope. The Ikelite Manufacturing Company makes housings for just about any camera,flash or compass.

Located in Indianapolis, Ikelite prefers that customers order through local dealers.

The company will accept direct orders, though, when accompanied by money order or cashier's check. And they're always glad to answer questions pertaining to underwater housings. Contact them by writing Ikelite Underwater Systems, Box 88100, Dept. A, Indianapolis, Indiana 46208.

Underwater Cameras. The alternative to buying underwater housings for your equipment. Unfortunately, the market is limited. The only widely available water-tight camera is the Nikons. A new one (35 mm format) lists for $407 with lens. Used models range anywhere from $150 to check the classifieds and camera stores selling used equipment.