i would NOT eat Mexican food ON the riverwalk - unless it was at Casa Rio - all the rest suck in my opinion
you're better off walking to El Mercado (spanish for The Market) -and eating at Mi Tiera or La Margarita (they are next to each other and owned by the same company)
Personally, I'm not a big fan of Casa Rio either. Your suggestion about Mi Tierra or La Margarita is a good one. I forgot about La Margarita- been a long time. It's a great place for sure.