Fish and seafood are a great source of protein. They are high in omega 3 fatty acids (DHA and EPA) and are a good source of iodine. (1)NIH. National Institutes of Health. Iodine. U.S. Department of Health & Human Services. Available here.
The healthiest fish in terms of its nutritional value are oily fish such as sardines, fresh tuna, trout, salmon, mackerel, herring, and pilchards. These fish contain more omega 3 fatty acids and vitamin D. In general, including fish in our diet has many health benefits (article on health benefits of fish coming soon..). Please note, however, that even though eating fish and seafood is considered healthy, its consumption should be balanced as for any other foods. Fish, especially larger and older predators, such as shark, ray, swordfish, barramundi, gemfish, orange roughly, blue fin tuna and ling accumulate heavy metals such as mercury.
Oily fish have many health benefits but can accumulate pollutants such as dioxins and PCBs (polychlorinated biphenyls). (2)Better Health Channel. Mercury in fish. State Government of Victoria. Department of health. Available here. (3)FDA – U.S. Food and Drug Adminstration. Questions & Answers About Dioxins and Food Safety. February 2012. Available here.
Eating fish 2 – 3 times per week from trusted sources (clean waters) and avoiding large predatory fish is my advice, based on the Australian and New Zealand government recommendations related to mercury contents in fish. In the case of pregnancy, large, predatory fish should be avoided. (4)NIH. National Institutes of Health. Iodine. U.S. Department of Health & Human Services. Available here.

References   [ + ]

1. NIH. National Institutes of Health. Iodine. U.S. Department of Health & Human Services. Available here.
2. Better Health Channel. Mercury in fish. State Government of Victoria. Department of health. Available here.
3. FDA – U.S. Food and Drug Adminstration. Questions & Answers About Dioxins and Food Safety. February 2012. Available here.
4. NIH. National Institutes of Health. Iodine. U.S. Department of Health & Human Services. Available here.