Let there be light! There is complete darkness in a cave; no sun beam comes through the rocks over your head, so you need a lot of light. I use underwater strobes, specially designed for UW-photographers – Ikelite DS160. Actually, I used up to 8 strobes simultaneously. Probably, you may think, it is not big quantity – well, just imagine that you cannot place them on tripods (because of silt on the floor). So you have to operate them holding them in your hands.
I love sunlight, but the best pictures of mine had been made with strobes’ light.