I did the Manta bowl/ whale shark thing in March this year.
I'll be honest - Manta bowl - as a dive site - sucks. It's boring, heaps of current which means a rapid descent - you need to get down quick to make sure the current doesn't take you out beyond the submerged island you are diving on. You will be hokin yourself on to rocks, and if you have done a few dives, you will use hardly any air apart form the initial descent. You basically sit around, hooked to a rock and wai. every 5 or so minutes you unhook, let yourself drift a bit then hook on again.
Basically as a divesite, I would avoid it - EXCEPT! - there are mantas and whale sharks - and if eithe rturns up, then it is one of the best dives you will ever have!
I dived it 6 times. the first dive last about 74 seconds - the other people on the boat didn't quite grasp the "quick descent" concept, so they didn't even make it down. 2nd dive - we saw a whale shark! Awesome! my other 4 dives on it were no shows for mantas or whale sharks.
Yet the group who dived an hour before us saw 5 mantas. Luck of the draw, really.
The DM I dived with say he has a theory that you will only really get mantas if the whale sharks aren't about, they very rarely see both. It IS worth diving san Miguel island which is not far away. It has lots of caverns, can be dived on all sides so there is always a protected spot, and is generally teaming with life and gorgeous coral!.
the whale shark interaction in Donsol is also fantastic - I swam with 14 of them in about an hour and a bit. The guys were quite happy to keep going, but I was truly exhausted.
anyway, hope you have fun! I loved it and would like to go back if I manage to get back to PI in march