I used to do a lot of underwater film photography - a few tips:

Get as close to your subject as possible - the water will act as a filter in bad ways. This means using either macro, or super wide angle. The two lenses that I used the most for underwater were a 50mm macro lens, and a 17mm wide angle on a 35mm body.


For macro, use a strobe, set your exposure before your dive, and don't worry about it after that - if I had a subject that was particularly bright, and I was afraid of overexposing with the flash - I would put a finger or two in front of the flash to reduce the exposure a bit.

Also, when shooting macro, I would carry a piece of black plastic in my pocket, and if shooting such that there was open water behind my subject, I would hold the plastic behind the subject (far enough back to be well out of focus). This prevented backscatter from bits of stuff floating in the water behind the subject, reflecting the strobe light back into the camera.

I did most of my photography in Southern California where the water is more brown/green than blue, when shooting with the wide angle lens, I would put a blue filter on my camera (the one made for shooting daylight film with tungsten lights), and the complementary red filter on my strobe. The result was that the foreground subject would have approximately correct colour, but the background would look blue.

I tended to use Velvia when using the wide angle lens, as it did a good job with the dominant blues and greens, but when shooting macro, many of my subjects were warmer colors and I got my best results with Kodachrome. Film choices now are more limited.