i've been staying at the Casa Del Mar for a week in each of the last 5 or 6 Decembers. It's been a group trip each time - anywhere from 8-18 people, all staying at the Casa Del Mar, all diving with Dive with Martin, and most of us gathering for meals at various restaurants. In general, I like Casa Del Mar. It has nice comfortable rooms - get a Superior Room, NOT the Standard, which are smaller, dingier, and overlook the parking lot in back; my personal preference is ground floor, so I can walk out my sliding glass door for a quick dip in the pool or to go to breakfast, while others prefer an upstairs room with a balcony to hang out their wetsuits to dry - up to you. It has a nice pool area and a decent breakfast, and is convenient to diving. When I've been there, Breakfast was included in the room price. Rooms do not have minifridge, which to me is a downside, unless you upgrade to the more expensive Cabana Rooms.
I have never used the Meal Plan there, and with Cozumel's wealth of good restaurants, I probably never will. You can walk to Downtown from CDM, but it is a hike - I'm guessing maybe 2 miles. It's a long walk. Taxi fares from CDM to Centro are 80 Pesos, about $5.50 US at current exchange rates. Since I go with a group, we always share the taxis, so per person costs are pretty reasonable. There are several acceptable restaurants in easy walking distance, like Ernestos's right across the street, Tiki, Paprika just a little ways to the north, and Blue Angel, a little farther north (maybe a 10 minute walk), even a Hardee's in the tourist mall across from the International Pier, and a pretty decent Chinese place just a little ways South of the International Pier - again, about a 10 minute walk. We ate most meals other than breakfast in town, as we have a list of favorites that we go to every year - Casa Mission, Guido's, El Moro for Sunday lunch (Cochinita Pibil - YUMMY!!), Pescaderia San Carlos, La Perlita, Cha Cha's Kitchen, Del Sur Argentine Empanadas, etc.