Culebra is not about cuisine at all. You get the basics, and they're usually done fairly well. Some recommended options:
LAS DELICIAS: specializes in lobster dishes and is a bit out of the way, but worth it for a meal or two. The owners are super-friendly and the restaurant is totally family-friendly.
OASIS: great pizzeria right on the main street in the town. Get some beers here and a lovely seafood pizza with squid, conch, octopus, and other fruits of the sea for a great deal.
EL CAOBO (or "Tina's"): find this place (just ask around) for excellent "locals' food," meaning fried fish, pork chops, and other Puerto Rican specialties. It's an extremely casual and very laid-back family-run business in the heart of "real" Culebra.
EL PATIO BAKERY: serves some decent pastries, but skip the sandwiches. The coffee isn't terrible, and it's very cheap. But see my Cafe Isola entry for better sandwich and coffee options.