Can't you see that the arch is too small for the sides of the boat to get through?
and at any moment another of those waves may come in.

It's all right, Ladle, if you'll only be firm." "I'll be as firm as you are," said the boy angrily.
"Then help me push her along." Mike pressed his hands against the roof, Vince did the same; and they both thrust hard, but in spite of all the boat did not stir.
"Why, you're pushing to send it in," said Mike.
"And you to drive it out! What nonsense! This place is sure to get bigger inside, where the water has washed it out.

We must get right in, beyond where the water rises." Mike shuddered; for the silence and darkness of the place would, he felt, be horrible, and all the time he knew that the water would be gradually chasing them, like some terribly fierce creature, bent on suffocating them in its awful embrace.
Vince's was the stronger will; and his companion yielded, changing his tactics, and forcing the boat along for some distance before there was any change in the roof, which crushed down upon them as low as ever, and Mike began once more to protest.
"It's of no use," he said: "we may as well be smothered where we can see as here, where it is so dark.

Let's go back as far as we can." "No; I'm sure this place will open out more if we go farther in." At that moment there was a loud, plashing noise far inward, and this raised such loud reverberations that Mike was fain to confess that the roof must be far higher.
Vince took advantage of this to urge his companion on; and a minute later they could not touch the rock above them with their hands, while a little farther on it could not be reached with an oar.
"Yes, it's bigger," granted Mike; "but we shall be suffocated all the same.

There can't be enough air to last us till the tide goes down." "We shall see," said Vince; and then, quite cheerily: "I say, this is better than wading, the same as we did in the seal hole." "Yes, but there are seals here.