Can a fish living in fresh water be called seafood?
Don't read too much into the "sea"; there's no rule that every word in the language has to stick precisely to its etymological roots.

Seafood just means edible aquatic life, i.e. fish and shellfish in general. It's a food word, not a biology word, and fish on your plate looks pretty much the same whether it's freshwater or saltwater fish, so generally it all gets lumped into one category.