Starfishman can regenerate and has a tough, armored exoskeleton, but he is not invincible! Here is a list of Starfishman's weaknesses:
Although Starfishman has an incredible ability to regenerate, he has never had the limit of this ability tested dramatically, so Starfishman is unsure if he is invulnerable. He has a fear of being chopped up into small pieces, as he thinks he would be unable to reconstitute himself, and lose his identity as an individual. It is possible that Starfishman could be killed if he were to be ground up into small pieces, or if his head was separated from his body. This is one of Starfishman's greatest fears.
Fear of Crustaceans
He is deathly afraid of crustaceans. He has covered for this fear by telling people he is allergic to shellfish, but the truth is Kino Marin had a traumatic accident as a child involving a crab. While visiting the beach with his mother, Kino reached out to touch a starfish and didn't see a large crab behind the rock. It reached out and pincered his finger, severing it at the middle joint. The finger was not recoverable and he lived with the disfigurement for years. He downplays the seriousness of the injury as it healed relatively quickly.
Starfishman's strength is the use of water, but it's also his weakness. If Starfishman is dehydrated or cut off from a water source for a long period of time, he is unable to use many of his powers. As a result, Starfishman prefers to stay underwater or close to natural water sources.