Actually, let me amend that...
Assuming you're using an ISO 100 film of some flavor, my rule of thumb is 1/60th sec at f4.0, 60 to 80 fsw in clear tropcal water on a bright sunny day.
So, I'd expect the same to be reasonable for a sillouette type image from 110 fsw.
In your shoes, I'd:
1)Arrive at shooting depth.
2)Set shutter speed to the slowest I feel confident hand-holding (for me, 1/15th with a 15mm lens, you might start with 1/60th)
3)Point the camera up toward toward the surface at the angle you'll be shooting. It's important that the view of the surface be unobstructed.
4)Play with the aperture knob until the over/under LEDs indicate a good exposure. This becomes your baseline exposure.
5)Set the focus so that the infinity mark is a bit inside the depth of field indicator. This sets the lens at hyperfocal.
6)Swim around to find the right composition.
7)Hold breath until bubbles are out of upward-looking image. Click.
8)Bracket 2 stops up and down.
9)Soup film and proudly show off award-winning image.
All the best, James
