I was there in Jan. 2006 and Feb. 2007 on Solmar V. Socorros is a great dive destination. The water gets colder in January - low to mid 70's.
I don't think that there is such a thing as a bad time to go. The mantas are always going to be there, and to me, it's one of the best manta dives because they are huge and seem to like to come close to the divers. Touching used to be allowed but no more.
I also had good experiences on both trips with bottlenose dolphins - quite friendly and willing to swim near the divers.
Creatures that you might see are scalloped hammerhead sharks, silvertips, Galapagos sharks, white-tip reef sharks and silkies. Saw them all on both trips. In the Feb. 2007 trip, we also saw a tiger shark. Whalesharks are usually October/November when the water is warmer. Maybe even December, who knows.
January is usually when the humpback whales start showing up. As you get further into the year (Feb. and March, and sometimes even into May), the sightings become more frequent. You will hear them while u/w and you will see them from the boat, but no guarantees that you will see them u/w. Sometimes their thumping is so intense that you feel the vibrations travel right thru your body.
I didn't have any u/w encounters on either trip, but some people on my last trip got a quick glimpse on snorkel when they were dropped into the water right next to one.