The "humans are fish" idea comes from cladistics.

"Fish", if taken as a monophyletic term, includes land mammals because tetrapods are osteichthyans -- bony fish.

In common use, "fish" is, however a paraphyletic group which excludes tetrapods but otherwise includes all other osteichthyans.

Since starfishes don't include tetrapods, nor vice versa (nor do they share biological features to be in a polyphyletic grouping like "crabs"), the relevant common term is "Animalia" -- "animal".