Real Image Viewing

The LCD monitor on the camera back lets you see your subject exactly as it will be framed on your photograph. This does away with one of the biggest disadvantages associated with entry level film cameras : the separate viewfinder. Separate viewfinders see, as the term suggests, a separate image to the lens. Overcoming this to get accurately framed images requires practice. Often heads or tails are cut off, or part of the picture is excluded or stuff you didn't want in the shot strays in. The LCD monitor avoids these problems altogether.

Instant Playback

The LCD monitor also lets you see the picture you have just taken seconds after you took it. This instant feedback lets you confirm that you got the picture first time or spurs you on to try again.
The instant playback also puts you on a fast track learning curve. You will quickly discover what works and what doesn't. You can see the effect instantly in the water. With film cameras it ís often hard to get your results processed on site. The delay in finding out how your pictures are turning out tends to hold peoples learning curve back. For many divers who will only get to use their cameras abroad a couple of times a year, this is a very real disadvantage to film.

Versatile Lenses

Digital cameras usually have zoom lenses built in. The zoom affords you the flexibility to frame shots the way you want and to zoom in on shy subjects. Keeping your distance from easily spooked critters makes it much easier to capture them on card. The close up or macro facility that is also built into most digital cameras allows you to shoot very small subjects like cup corals or clown fish easily.
Autofocus is as reliable underwater as it is in air. It usually allows you to shoot subjects at any distance from a few centimeters away to infinity. If a subject moves away or towards you the autofocus can normally track it.
Entry level film cameras are restricted in their choice of lenses and how they can be used. In turn, this restricts the kind of picture and subjects that you can take. Most entry level film cameras have a semi wide angle lens fitted as standard. They may be fixed focus, typically operating no closer than 0.5 of a metre. This is too wide a lens and too distant on minimum focus to fill the frame with small animals like many reef fish. Some lenses have to be focused by guess. This should yield a sharper image, but creates additional problems. You need to guess the distance accurately and it takes time to do. If both you and your subject are moving it becomes even more difficult to get right.
Only a small number of entry level film cameras accept wide angle lenses. Without access to these lenses many subjects are off limits including close focus wide angle pictures, large subjects like Whale Sharks and Mantas and anything but close ups in low viz.
Most entry level cameras do accept close up and macro lenses. Normally these use framers to delineate the area that will be in the picture. These have to be placed over the subject. Not surprisingly many animals will not tolerate these and flee. You are also restricted to shooting this one image size. There are restrictions on how close you can get, precluding very small subjects that are easily handled by digital cameras

Advanced Technology

Underwater film cameras tend to be pretty basic. They occupy a small, niche market. There is little money for development and consequently these cameras have lagged behind their terrestrial counterparts. Digital has widened that gap even further.
Taking advantage of the volume marketplace has enabled digital camera manufacturers to increase specifications while actually reducing prices to the consumer. So many digital cameras offer a choice of exposure modes, such as program for getting started, alongside shutter or aperture priority or full manual for those who wish to take more control. You'll often get exposure compensation for overcoming tricky lighting conditions and a choice of flash modes including slow sync for introducing movement to your images.

Control of the Final Image

Shooting prints has normally placed the photographer at the mercy of the developing and printing lab. Some are very good indeed. Others are not. The colour cast in underwater shots often seems to confuse automatic printers and the final results can be lackluster.
Digital photographers can control the look of their images by using software such as Photoshop. Using simple programs you can alter the brightness and contrast of your images, enhance or change colours, remove or add subject matter and crop to your personal taste. They can also print their own pictures inexpensively.

Low Shooting Costs

Digital is a very low cost way to shoot underwater pictures. Storage media, unlike film, can be used over and over again. It costs only electricity to view your results on your computer. Film has to be processed, which can seem quite expensive for the number of shots you are likely to want to keep.
You also have almost unlimited shots per dive. You can literally take hundreds of pictures on a single card. With film you are typically limited to 36 frames. If you want more pictures you need to take additional cameras down with you. With so few shots available, film photographers have to think very carefully about taking risks. The digital photographer has no such concerns. You can try for a shot that might not work out and simply delete it if it fails. But equally you might end up with a truly excellent image.