I am sorry to have to disagree with some points made by rramaley.
First; you need a camera that is very good in low-light. Flash is forbidden with thresher (with reputable dive companies) as they are very succeptable to bright light. One flash and you will not see the shark again on your dive - plus you will be highly unpopular with everybody from all companies there.
Do you have DSLR with choice of lense? If so, choose something like a zoom macro 18-50/55 F2.8 or similar. You will shoot around ISO 400-800 minimum. Visibility can be OK, but assume 33ft/10 meters.
The shark will come close to you if you dive in a small group and remain still. Better to dive with a good operator who understands this. Try divelinkcebu.
Manta are not aggressive!!!!! It is wrong to describe these majestic animals like this. Again, shoot 400/200 ISO - normally manta visit in the afternoon with better light.
They come to Monad for cleaning and, yes, some bear scars of their life in the ocean. We have seen plenty manta who bear scars of their lifes including giants with nets on them (sad but true) worldwide.
Hope this helps.