He's definitely not unkillable, especially before he gets any resistances built.
But you have to respect his all in potential w/ hook > Ult. My favorite instance of this where i was nautilus vs a yasuo and he tried to stay in the middle of the lane and farm while he was a bit above 1/2 hp, so i worked my way around the minions, hooked him after he did a dash, then ulted him to secure the kill.
If you arent playing a more tanky toplaner, such as renekton, you really have to respect the naut all in, remember tanks have higher base damages and poor scalings so they are still hitting w/ some force even when building full tank.