I'm terribly sorry for my absence, as I have been busy with a sports tournament.
But, pressing on: you solved it, Thalia!
Now back to our story-
Nervously, you mumble, "A king?"
With a shout that makes your eardrums pound like a marching band, Sting (the king) congratulates you on your riddle-solving ability. "Feel free to pass!" He booms. Relieved that Sting did not live up to his name, you and the merman swim on. Unfortunately, you know well enough that another danger is not far off. Because you are on your guard, the trial is not unexpected. As you and the merman swish peacefully deeper into the ocean, you become vaguely aware of a dark presence.
It gradually becomes more pronounced (and all the more terrifying in its clarity.) A massive shark is making a beeline (or perhaps sharkline) for its prey...you. Quickly, the merman hands you a small handheld device. "This is an underwater item propeller. It can shoot objects at a very rapid rate underwater. You must use it to defeat this beast. I cannot help you other than with this. Please, stop this monster!"
You have a knife you could put in the propeller, but the shark is so big that it will do hardly any damage, even to an eye or another sensitive spot. How do you defeat the shark?
Go with haste! Once again, your life is at stake!