Going on Carnival Jubilee this July, it's taking us to Cozumel, Costa Maya, and Roatan Island. How bad is the seaweed down there?

I've seen many vids, and Costa Maya looks horrible. Will there be a chance to see crystal clear blue water (I've never seen it before, only Galveston's murky water)? Also, where is the best place to do excursions? One more thing, since the beach will most likely be bad at Costa Maya (and maybe other places too, idk), what would be the best thing to do there?