When you're in a country like Mexico with so much coastline, you can expect to eat some killer seafood.
Shrimp, camarones in Spanish, is widely available. Shrimp tacos are often deep fried (empanizado) and usually include avocado, lettuce, tomato and salsa. Another great way to eat shrimp is in a cocktail (coctel). Full of hot sauce, cilantro, onion and slices of avocado, these are good with beer. Try pouring a little in before you finish it off.An empanada is stuffed with shrimp, fish or almost anything else and then deep fried. You can also get tostadas, which are a big flat hard tortilla with lettuce, onion, avocado and the seafood on top.
Ceviches are like seafood salads and often include shrimp, fish, octopus or some other seafood. Unlike in Peru, which is the source for original ceviche, Mexican ceviches have cooked fish, rather than raw. You can also order a seafood salad (ensalada de mariscos), and it will be similar but will have more vegetables.
Fish comes in many ways. I like to get the whole fish cooked up so you can eat every bit of meat, picking it off the bones, but if you don´t like the food staring up at you as you eat it then you can order a filete. I'm sure you can guess what the translation is.
Finally, on the beach or in a restaurant, you should always be able to find pescadillas, which are quesadillas with fish (pescado). They should be cheap and ready to be doused in hot sauce.
There's a lot more seafood to eat in Mexico, but hopefully these are enough suggestions to get you started. Provecho!