Starting from October things would be getting better and better in terms of diving.
Atleast at the area where all the boats normally cruise.
As you will jumping on a liveaboard the guides should be able to find mantas,whale-shark. Cause you wont be sticking only to one atoll. If you dont find it on the first atoll, you will be heading towards the next to find them. So you will have a good chance of finding them. But ofcourse its never a 100% as you are playing with nature here. But chances are good.
If you are thinking of Nov or Dec then it might be better to come on Dec..Not much of a difference but better chances.
Before you book the Boat just ask them about the route they plan to have.. Throw some questions at them. If they are planning to be at Vaavu Atoll and Meemu atoll. You dont have a big chance of seeing the Mantas or the Whale-shark. As these two atolls have mainly channel dives looking for Rays,grey,white-tip, etc...Vis should be good at these atolls at that time cause you will be diving most of the time at the Eastern border of these atolls.
But Ari Atoll Western Border/North Male' Atoll Western Border should have Mantas and Whale-shark at this period. Vis wont be great because of the plankton. Ari Atoll is good for both Mantas and Whale-shark but the North Male' Atoll is limited to find Whale-shark but Mantas could be found on a few cleaning stations.
Marine life if pretty good in all the atolls. Everywhere you jump you will have enough of fish to see. Loads of stuff..
Its only when you are looking for a certain fish you have to be carefull in selecting the area. Otherwise on average most of the dive sites gives you lots to see,specially if you like both BIG and small stuff.
Hope this would HELP