Eating undercooked or even raw fish is usually okay for most people especially if the person is already in good health.
But eating undercooked fish or raw fish can sometimes cause illness.
Some potential illnesses and health problems that can occur from eating undercooked or raw fish can include Salmonella and Vibrio vulnificus.
But as long as the fish is partially cooked there shouldn't be much of a problem but if you do begin feeling sick as a result of eating the undercooked fish or raw fish then you need to go to urgent care or the emergency room for treatment just in case it caused food poisoning that could be deadly.
Personally I like to make sure my fish is cooked properly and all the way through because I cannot stand to eat undercooked fish or even raw fish for that matter.