As long as the fish has remained frozen up until the time you cooked the fish then the undercooked fish is usually not dangerous to eat and should not cause you any health issues.
However it's best to cook the fish all the way before eating it to kill any bacteria that could be present on the undercooked fish.
A lot of fish can be eaten raw but you do risk getting food poisoning even from fish that can be eaten raw because parasites and bacteria can be on the raw fish that you cannot see.
Then you could get food poisoning.
I would always recommend cooking the fish all the way through before eating it.
But if you do eat undercooked fish you should be aware that you could potentially get food poisoning as a result.