Fish is excellent as are most marine diets but some north west europeans might eventually die if they went onto an eskimo diet. By no means all, but some. You can have too much of a good thing and genetically, we are adapted to a different diet. It's probably best to mix and match lots of fish with other things. Marine, riverine, esturine and lacustrine diets contain most of those vital things for humans, iodine in the salt, calcium in small boney fish, various rare but vital vitamins etc etc. But, they do other things too, like thin the blood which may be a good thing but which also be a bad thing, depending on your own genetic make up. Mixing and matching, keeping things varied is playing it safe. Fish goes very well with tomatoes, broccoli, olives, onion, pulses but so does chicken and ham. Vary the type of fish that you eat and don't overlook things like mussels which are becoming easier to obtain. King prawns are my favourite. They go well with tomatoes and some wholewheat pasta. So does tuna, just add some sliced olives into the sauce.