I did wreck and reef dives w scubadivingcancun.com (km 3.5) and was impressed with their staff. (Alberto was my DM) They have a smaller open launch type boat (with some shade) back roll off the side, removable ladder on the side.
I don't know if they do denotes or Coz.
Highly recommend Cocos and Grill at around the same location (I think it's called Marina Chac Chi or something. Casual beachside dining, but very yummy. Grilled octopus was yummy. Sirenas by the Ferry to Isla Mujeres was pricier, but good raw seafood. The atmosphere is dockside and nice once the nearby pirate show raises anchor and you can see the pirate ship in the distance, as opposed to hear them screaming and rah-rahing nearby