...he ordered a seat to be placed for him on the sea-shore when the tide was coming in; thus seated, he shouted to the flowing sea, "Thou, too, art subject to my command, as the land on which I am seated is mine; and no one has ever resisted my commands with impunity. I command you, then, not to flow over my land, nor presume to wet the feet and the robe of your lord." The tide, however, continuing to rise as usual, dashed over his feet and legs with out respect to his royal person. Then the king leaped backwards, saying: "Let all me know how empty and worthless is the power of kings, for there is none worthy of the name, but He whom heaven, earth, and sea obey by eternal laws." From thenceforth King Canute never wore his crown of gold, but placed it for a lasting memorial on the image of our Lord affixed to a cross, to the honour of God the almighty King; through whose mercy may the soul of Canute, the king, enjoy eternal rest."
From The Chronicle of Henry of Huntingdon, p199, translated by Thomas Forester, accessed at Openlibrary.org.
Anyway, apocryphal or not, James Pegrum built an amazing LEGO rendition.