Regarding "fish" Old classification relied on "phenotype" characteristics. And yes lamprey, a shark or a cod has little in common genetically. But they still share some common traits that distinguishes them considerably from whales, sea lions and seals. I still think the term "fish" is useful, and modern classifications rely more on genetics so I would say that the argument is semi void.

actually no such thing as fish

ELI22 undergrad degree?

Source (Tumblr), alternate frontend

"What is a Fish?" coming soon

Whales are fish, fite me