Firstly, you’ll start on a Toad spaceship, and from there you need to use the Launch Star to take you over to the Kingfin’s lair. To defeat the Kingfin you’ll need to throw shells at it. The red shells will help you out if your aim is a bit off. You’ll also need to swim through bubbles regularly to ensure you don’t run out of air.
When you first land a successful blow on the shark, he’ll spawn in two Skeletal Fish Guards. If they see you, they’ll swim at you in an attempt to hurt you, but they’ll explode if they hit the ground or an obstacle so they’re relatively easy to avoid.
After you’ve hit the Kingpin two more times, he’ll begin to spawn a lot more fish guards. Be careful, as there will be a lot of fish guards swimming for you now, making it trickier to hit the Kingfin.
Once you’ve defeated the Kingfin, a Power Star will spawn above a hot spring.