You can eat any reasonable quantity, every day if you like, preferably cooked by broiling, poaching, baking, stir-fry or grilling. If you like, cook with a small quantity of olive oil. Experiment with the creative use of herbal marinades made with garlic, thyme, or Cajun spices.
Beer makes a good marinade for fish.
Avoid breaded fish, since the fat retention and starch content add unnecessary calories. Sushi can be an excellent way to eat mackerel, salmon, and tuna in their healthiest uncooked form. American-style sushi restaurants are opening all across the nation. Beware of mayonnaise-laden tuna, shrimp, or lobster salad. If you do make it at home, use sparing amounts of low-calorie mayonnaise.