I have seen them all year round. There are less divers and therefore less boats during the off season(May/June-Sept./Oct) so you might get more chances for one on one encounters without diver interference. Most resorts/operators will know when to go with snorkelers so maybe contact the resort. There are many other places to see mantas in that area so you might have other options as well.
Another option to consider is adding on a few days in Bali or Nusa Penida area to snorkel with the mantas, but they are mostly reef mantas there...so you need to learn to dive then go to Socorro!!!