Silly Christians keep reading the wrong book! Here's the full explanation of tides:

"The king called to Thor, and asked him what he thought he could best do to prove himself as mighty as the stories told of him. Thor answered that he would undertake to drink more mead than any one of the king's men. At this proposal the king laughed aloud, as if it were a giant joke. He summoned his cup-bearer to fetch his horn of punishment, out of which the giants were wont to drink in turn. And when they returned to the hall, the great vessel was brought to the king.

"When any one empties this horn at one draught, we call him a famous drinker," said the king. "Some of my men empty it in two trials; but no one is so poor a manikin that he cannot empty it in three. Take the horn, Thor, and see what you can do with it."

Now Thor was very thirsty, so he seized the horn eagerly. It did not seem to him so very large, for he had drunk from other mighty vessels ere now. But indeed, it was deep. He raised it to his lips and took a long pull, saying to himself, "There! I have emptied it already, I know." Yet when he set the horn down to see how well he had done, he found that he seemed scarcely to have drained a drop; the horn was brimming as before. The king chuckled.

"Well, you have drunk but little," he said. "I would never have believed that famous Thor would lower the horn so soon. But doubtless you will finish all at a second draught."

Instead of answering, Thor raised the horn once more to his lips, resolved to do better than before. But for some reason the tip of the horn seemed hard to raise, and when he set the vessel down again his heart sank, for he feared that he had drunk even less than at his first trial. Yet he had really done better, for now it was easy to carry the horn with-out spilling. The king smiled grimly. "How now, Thor!" he cried. "You have left too much for your third trial. I fear you will never be able to empty the little horn in three draughts, as the least of my men can do. Ho, ho! You will not be thought so great a hero here as the folk deem you in Asgard, if you cannot play some other game more skillfully than you do this one."

At this speech Thor grew very angry. He raised the horn to his mouth and drank lustily, as long as he was able. But when he looked into the horn, he found that some drops still remained. He had not been able to empty it in three draughts. Angrily he flung down the horn, and said that he would have no more of it."

(...)

"When you drank from the long horn, thinking you had done so ill, in truth you had performed a miracle,—never thought I to behold the like. You guessed not that the end of the horn was out in the ocean, which no one might drain dry. Yet, mighty one, the droughts you swallowed have lowered the tide upon the shore. Henceforth at certain times the sea will ebb; and this is by great Thor's drinking."

(For the full story, go here.)