As far as food borne illness, I'm a fan of having a meal prepared for me as opposed to eating buffet food. The only time I had a problem with food in Mexico was in Playa del Carmen at what was the newest, nicest, freshest AI south of Playa. Besides the buffets, there were 5 additional restaurants you could book. Timing is everything, and had I waited 5 minutes I'd have avoided the likely culprit. (One of our group came to the table right after I'd eaten and said "Wow, did you see all the flies on that roast beef? There must have been 50 of them." Well I hadn't see any flies when I filled my plate. The beef tasted fine. I was fine, until about 4 hours after eating. 8 hour after eating I was projectile vomiting.) The food in Mexico is fine, but no matter what country you're in, if the food sits around at the wrong temperature for long enough, it'll go bad.
My last experience with food poisoning was in Cozumel. I was on a boat with a guy that was staying a one of the AIs mentioned above. He was throwing up under the water, on the boat, on the SI, and all the way back to his hotel from the Caletta. We pulled over six times for him to be sick. He'd been there for two days, and had only eaten at the hotel, and only at the buffet. The next day he was fine. It probably wasn't that the food was bad to start or incorrectly prepared but depending on the foodstuff, if it sits too long, it'll make you sick. The chance that a fresh, sizzling plate of fajitas is going to make you sick isn't comparable to the stainless pan filled with chicken and onions that may have sat there, being added to, for hours.
BTW, the hotel where I stay, Casa del Mar, has a breakfast buffet and I've had almost every breakfast I've eaten in Cozumel from that buffet with never a problem, but I'm always there right away in the morning, and the hotel is small. The food doesn't sit long enough to go bad. They don't have to prepare truckloads of food like at a huge resort. CDM also has an all inclusive plan, for something like $39/day per person. You order off the menu for your meals though, it's not buffet. I've stayed in a room that's as far away from the restaurant as you can get, and it takes less than 1 minute to get from the room to the restaurant. I timed it.
In short, if you stay at a smaller place with a buffet, you're probably less likely to have food that's been sitting around, or if you're at an AI where you order all your meals from a menu, there again, you're probably ok. Or for instance, my understanding is that at Scuba Club, you tell them ahead of time what you'd like for dinner, and/or lunch. I'd be comfortable with something like that. Not trying to start an AI versus not AI thing. They serve their purpose for some and in some cases, even if you're eating buffet food, it can be just fine. Huge hotel with huge buffets? Not my cup of tea. Never stayed at El Cid, but I've stayed for years at Casa del Mar, which is about a block away. The El Cid is pretty big as far as I can tell. The Park Royal looks huge to me. Both of them are very close to CDM. In addition, there are no fees imposed for CDM boat pick-up and they have an onsite dive operation, (Del Mar Aquatics), and easy shore dive access. I don't dive with Del Mar Aquatics, so I can't speak to that.
-Blair