Yes, according to the eighteenth century Linnaean classification. However, in Halakhah (which predates modern classification systems) water animals are all classified together (hence mollusks tuna are both "seafood").
If it comes from the water it requires fins and scales. They're also ready to eat once removed from the water, not requiring ritual slaughter.
Hmmm, then sea turtles should kosher - they have fins and scales.
Does that make no difference?