You're always going to get confusing information because obviously some people have different levels of experiences and expertise. As I always say, what one person loves, another will hate.
I'm a scuba diver with over 150 dives and have my open water, advanced open water, and rescue diver certifications. I've dove in many places around the world and snorkeled in many more. My wife can't dive because of ear problems, so we usually snorkel more than I like and I get to dive less than I want.
We've been to Cozumel about a dozen times on cruises plus we've stayed there a few times on vacation; a couple of times for 4 days, once for a week, and another time for 2 weeks. In fact, my first two scuba dives were done in Cozumel, which is where I fell in love with the sport.
Quite frankly, I can only say that those who say the snorkeling is really good either have no other place to compare it against or their 'wow' factor is very low.
As I mentioned, the coral reef in Cozumel doesn't start until around 45' deep. The majority of exotic fish stay around the reef system because it provides food and shelter. While some fish can be found in shallower areas and there are a few places where minor reefs can be found, if you've never snorkeled before, this can still be exciting and interesting. It doesn't compare to places more suitable for good snorkeling, but fun nonetheless.
Like I said, while it's not bad, it's not really all that great. Personally, one of the best places in the area we've ever seen for snorkeling is West End Beach in Roatan.
Labadee has some snorkeling that's okay - again, not great. But for beginners it's fun.
We've not yet been to Falmouth, so perhaps someone else can provide some information for you on that port concerning the snorkeling areas. We've been to Ocho Rios numerous times and spend the day at an all-inclusive resort that has some nice snorkeling - again, not great. Maybe Falmouth is better.