Hi Darren,
I will pick up a couple of your questions but avoid those that are D100 specific as I don't have the experience with that camera.
1: No bid - see above.
2: It is not a time related thing as in this case you are not adjusting for a camera dynamic or drift. I have stored WB settings and used them a year or more later.
3: Yes, if I were doing a dive and taking pictures at every level between 0 and say 18m then I would WB at 0, 6, 12 and 18 or so.
You don't need to carry white cards and all that fancy stuff - just use a sort of white or greyish area to set the balance - the back of your hand or sandy bottom work fine for me.
One issue I have had is my camera (D70 initially and now D200) rejecting the WB 'picture' - this has pretty much come down to underexposure of the WB shot.
4: No, but I genuinely believe that you will get better pictures with one than without. Having said that there are some inherent issues that you need to work with when you use a Magic filter.
Shoot RAW if at all possible - it will allow you to 'recover' shots in post that will be headed for the round bin if you shoot jpeg.
If you shoot RAW then the WB at the time of taking the shot is irrelevant, it is still worth doing the WB however so that you get a true picture on the back of the camera when you review.
The Magic filter will drop you 1-2 stops of light.
Try to shoot with the sun behind you and with a slight downward camera angle.
The filter is good to about 18m max - after that you will find that the exposure time give you a problem and that the correction that the filter provides is not ideal below this depth.
The Magic filter is a great tool and can be a lot of fun - conversion to B&W also works well from Magic filter shots.
Enjoy.
Paul C