If its 5e, he can hold his breath for 1min + a number of minutes qual to the constitution modifier, which should be at least +2. That 30 rounds, with a lv 2 heat metal that's 60d8 or a 270hp average, with a lv 9 heat metal that's 270d8 or 1215 average. Water breathing spell is also super easy to get though one could say the inside of the kraken isn't exactly water.