In that case, I'd suggest you try Bodega, which is outstanding (very casual, though). However, Bodega is Cuban, not Mexican. (The other outstanding Cuban restaurant in St. Pete I can recommend is La Segunda Bakery, which is a spin-off of the original in Ybor City. Of course, the granddaddy of all foods Cuban in Tampa proper is the original Columbia restaurant in Ybor, but that's much more upscale. (Excellent, but upscale.))
I'm not a huge fan of Mexican food myself, but I've been told that Agave Lounge in SPB is very good. It's a bit of a hole in the wall; if you really like Mexican food, that might appeal.