Hi Dg
I haven't used Magic filters..I have used red filters- they need certain water/sunlight conditions to get the best out of them. (c.f. Alex- I think there are many threads in the forums on this subject)
I did this shark dive one year in June- it was a cloudy day- light levels were low- viz was good- but not perfect. Also depth was about 70 feet.
I didn't have a camera with me on that occasion...and wished I had!
I cant comment on your optical set up .....so the next bit of advice is with a grain of salt... if I was doing the dive again with my rig...well.....
If you were to try with a filter- then you will def. need to bump up the iso big time, and maybe set your camera to speed priority to keep your shark shots sharp as opposed to blurry.
Would suggest you do a trial run/dive with your gear first- and see what is best- BEFORE- you do the shark dive- that way you can be totally ready for the sharks and not have to worry about messing round with camera settings and magic filters.
Now with a strobe- well you dont have to worry as much.
If I was in your shoes- I would forget about filter and take my strobe- period.
I would rather have my lovely sharks lit up and sharp- and believe me- they will get close enough for you to do this- effortlessly.
Hope this helps- and enjoy your dive!
D