Spent a Sunday snorkeling off Tortuga Island with Zuma Tours and it was insanely crowded. The tour itself was fine; the main guide was very friendly despite a rather annoying shtick that he stuck to when pointing out the sites from the boat, and everything ran on time and seemed safe enough (although the boat driver was clearly new). It was all pretty touristy, but it's a touristy area.
The issue came when we got to Tortuga. There were two snorkeling spots, both of which were completely over-run with other boats. Even if the crowd had been thinner, the water isn't particularly clear and there wasn't a whole lot to see.
After less than an hour of unremarkable snorkeling, we pulled up to a beach that would be a remote paradise were it not crawling with school tours, church tours, youth group tours, seniors tours, and every other conceivable tour. There was almost no room on the sand to stretch out, and the water at the beach was filthy from all the boats anchored there.
I don't know what the other snorkeling options are in Montezuma, or if the experience is notably better on weekdays, but I wouldn't recommend it on a weekend.
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