There is a wide choice of restaurants on Isla Mujeres, most of them provide excellent service and many offer really good value for money. The general advice however is don't eat in the street. It's always tempting when in Mexico to enter into the spirit of things and buy food from one of the many street vendors. However, those who prepare and cook food from stalls rarely have the right washing and cleaning facilities and one of the worst thing that can happen when you are holiday is to get an upset stomach and be laid up in bed whilst everyone else is enjoying themselves.
Somewhere that is worth dining at least once, if the budget will stretch to it, is Villa Rolandi which is the hotel in Sac Bajo, it's a lovely location and really good food. However the daily room rate for the hotel is $888 USD so don't expect it to be cheap.
Hidalgo is the main street on Isla Mujeres and Rolandi also have a less expensive restaurant on Hidalgo. It's a great place to eat because the food is good the service is excellent and if you have children it's ideal because they also serve Pizza. Further down Hidalgo on the right is Los Amigo's which always have fresh fish and their pescado cooked in salt is excellent, the wooden seats are a bit hard and upright.
Dopi's restaurant further down on the left hand side of Hidalgo is certainly worth a visit, the food is cooked by the restaurant's owner, and is always fresh and beautifully prepared.
Many people's favourite is Mama Rosa, the owner is Italian, it's air conditioned inside although the nicest place to sit is on the terrace. The service is faultless and neither can you fault the food; the fish is always fresh, there is a wide choice of meat dishes and of course again, very important if you have children, they also serve Pizza. The restaurant has real class.
It's always worth taking a mosquito repellent when you are dining out in the evening; one of the mosquito's favourite spots is under restaurants tables where they will relentlessly bite your ankles.