Bring water to boil in a large pot. Add the corn cobs and cook five minutes. Remove cobs and rinse under cold water. Stand the cobs on a board and scrape the kernels off with a knife. Transfer the kernels to a mixing bowl, with all remaining salsa ingredients. Combine thoroughly.

For tuna: Heat the oil in a heavy skillet. Season the tuna with the salt and pepper. When the oil is very hot-smoking, add the tuna. Sear about one minute on each side for medium rare. Let cool just a few minutes and slice thinly against the grain.

Arrange corn mixture on platter with sliced tuna on top or around. Serve at room temperature.