He is quite harmless.
But he might run against the yacht.
He might: and so might a hundred things happen which never do.

But I never heard of one of these whales running against a vessel; so I suppose he has sense enough to know that the yacht is no concern of his, and to keep out of its way.
But why does he make that tremendous noise only once, and then go under water again?
You must remember that he is not a fish.

A fish takes the water in through his mouth continually, and it runs over his gills, and out behind through his gill-covers.

So the gills suck-up the air out of the water, and send it into the fish's blood, just as they do in the newt-larva.
Yes, I know.
But the whale breathes with lungs like you and me; and when he goes under water he has to hold his breath, as you and I have.
What a long time he can hold it.
Yes.

He is a wonderful diver.