Well it would be pretty hard to catch a large amount of saltwater fish without technology such as sonar, massive boats with freezers etc.
also theres not that much fat, I would eat kilo's of the stuff when I first started the diet and was still hungry, I got rabbit starvation and would anger quickly. Theres not much fat on cold water fish let alone tropical fish. I know neolithic tribes ate a lot of saltwater fish and probably didn't have too much trouble catching them it's just you would have to have rice, manioc or taro or something like that or you would starve to death! So I think agriculture would have to be a must if eating marine fish I can't see how it would be possible without it.
also the Eskimos preferred mammals to fish.
Andrew