is lobster a fish or a sea food only?

According to wikipedia: Lobsters are invertebrates and have a tough protective exoskeleton. Like most arthropods, lobsters must molt in order to grow, leaving them vulnerable during this time. During the molting process, several species may experience a change in color. Lobsters live on rocky, sandy, or muddy bottoms from the shoreline to beyond the edge of the continental shelf. They generally live singly in crevices or in burrows under rocks. Lobsters typically eat live food, consisting of fish, mollusks, other crustaceans, worms, and some plant life. Occasionally, they will scavenge if necessary, and may resort to cannibalism in captivity; however, this has not been observed in the wild. Lobster skin in the stomachs of lobsters has been found before, although this is because lobsters will eat their shed skin after molting.[1] Lobsters grow throughout their lives and it is not unusual for a lobster to live for more than 100 years.[2] They can thus reach impressive sizes. According to the Guinness World Records, the largest lobster was caught in Nova Scotia, Canada and weighed 20.14 kg (44.4 lb).

Lobster ia a crustacean, not a fish. But since lobster comes form the sea, then yes, it is a seafood. So is seaweed. There are many types of seafood that also aren't fish or crustacean for that matter. Oysters, clams, mussels for example.