The chair lift is at Mahogany Bay. It's just like a ski lift chair. I'm not sure you'll be porting there, but if you do, it will take you from the terminal area to the beach area. There was a line there when we were there in May, so we just walked. It's not that far. There was decent snorkeling and a nice beach area. Carnival spent 70 million dollars creating the terminal/port/beach area. There are a few shops there and a great place to buy gelato. There's also a Fat Tuesdays. On the beach area, there are a few restaurants and bars, plenty of chairs and umbrellas, a volleyball court, restrooms and showers.
We also did a snorkel through the ship in Roatan that was excellent. One of our members recommends Foster's Beach. I wanted to try Tabyana beach since it was also highly recommended by one of our members, but they wanted $80 for the cab fare. They charge by cab, so you can cram as many people as you like into them to reduce the price per person
We, too, normally go to Chankanaab Park in Cozumel or Paradise Beach. I personally prefer Chankanaab, mainly because the snorkeling and beach area are just a little nicer. Paradise is nice, though.
Costa Maya we might go to the ruins or maybe just the beach. Haven't decided.
Hope this helps a little.