Boca Del Rio is the glitzy, upmarket beach district about 0.5 hours south of Veracruz. Those who don't mind parting with some serious pesos may choose to stay in one of the resorts down here; I, on the other hand, stayed in Veracruz, and caught the cheap and frequent bus service there and back.
There are several beaches in this area, such as Playa Mocambo, and Playa Villa Del Mar. But the best one, and the one that I spent much of the day at today, is the last one before the Boca Del Rio (lit: "Mouth of the River") itself, which I guess is called Playa Boca Del Rio. This beach isn't so great by Australian standards (or even by Mexican standards, from what I hear), but compared to the other beaches in the area, it's amazing.
You can walk all the way to the end of the beach, climb the few rocks at the southern end, and see the river letting out onto the ocean, and the continuing beaches on the opposite shore. This geography reminds me very much of Hawk's Nest, which is about 3 hours north of Sydney in Australia, and which has a river separating it from Nelson Bay.
Boca Del Rio is a great place to have a swim, to relax and read a book, and to enjoy the Mexican sun. But clearly, the Caribbean Gulf is not the place to go if you want to experience the best of Mexico's beaches.