Seafood? Tilapia are freshwater fish...
Picking words apart and applying literal interpretations to their component is often a bad idea. Language rarely works that way.
Cambridge English dictionary: "animals from the sea that can be eaten, especially fish or sea creatures with shells"
Merriam Webster: "edible marine fish and shellfish"
These are much more reasonable definitions, more line with modern usage. I doubt you will find any dictionaries that mirror your Britania definition.
I couldn't think of a better word that refers to cuisine involving animals that live in the water.