While mundane fire can't burn underwater, the sudden appearance of magically produced fire underwater would cause instant vaporisation, filling the affected area with bubbles of scalding hot steam. The sudden increase in water pressure from the vaporisation would have an impact like a small bomb, increasing the amount of disorientation and potential damage. In engineering, this is called cavitation and is significantly damaging to machinery. It's the reason a mantis shrimp can kill its prey with a punch that doesn't hit.

TL;DR: you can absolutely kill someone with an underwater fireball. Though you might want to convert half the dice to thunder damage.