For seafood restaurants I like El Conchita del Caribe. Fresh, tasty and reasonable prices.
In the interest of conservation in case there are people that just gotta have what they want, at the very least PLEASE, PLEASE, PLEASE respect the "closed season" dates that are supposed to be observed by law in Quintana Roo
Lobster - March 1 to June 30 (and this includes the mini-lobster caught down in Punta Allen on the mainland too)
Grouper - Feb 15 to March 15
There are also dates for Conch and Octopus, although I don't think it's too common to even get local conch anymore
Conch - May 1 to Oct 31
Octopus - Dec 15 to July 31
If you see these items on menus or specials during the closed season, don't order them. Also, please question management as to why they have them... they're either getting illegal local catch or you're paying for imported and/or frozen items. Why bother? You can get that at home.
If you want lobster, support the Maine lobster fisherman. Many of the boats shut down early in their 2008 season because the market got so bad they weren't able to make enough money to cover the cost of operations, which were much higher for them.
For seafood consumption in general, this is a great reference
The Ocean Project