True Ed, sensitive film is a must. (I thought it would be a must but it doesn't seem so) Thing is the amount of light changes depending on the depth. Light tends to absorb many wavelengths making everything look blueish (especially all the warm colors of the spectrum like red, orange and yellow).
To get good colors, you also have to be quite close to the subject. In the Time Life encyclopedia, they say that if you have 15 meters of visibility, your subject should be only 3 meters from you maximum (remember 5 to 1 ratio).
I don't know if it still in production but there used to be special underwater emulsions made for divers in order to counteract the effects of uneven light absorption.
But if you stay under 5 meters depth, you should be OK and not worry much about these considerations.
I did find this in the Photographer's Handbook which I think answers your questions about film sensitivity and exposure:
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