These famous lines from "Julius Caesar" exemplify an extended metaphor of the sea. (A metaphor is a figure of speech in which one thing is spoken of as though it were something else. An extended metaphor carries the figure of speech through several lines.)
This passage is particularly significant for its interplay between fate and free will. While fate presents the opportunity before them of winning a battle, Brutus speaks of choice that man has when presented with this act of fate. In Act IV, scene 3, as he urges Cassius to meet the advancing armies in Philippi. Arguing against Cassius's position that they should remain where they are and let their enemy come to them, Brutus says that their forces are in unfriendly territory, at the peak of their strength, and they must seize the opportunity before they weaken. He tells his brother-in-law that in men's lives there is a "tide" that will carry one out to the sea of success, greater adventure, power, etc. if one will ride this tide. But, if one allows this tide of opportunity to escape him, he will live a different life--one of mediocrity and regret,
Omitted, all the voyage of their life/Is bound in shallows and in miseries. (IV, iii, 219-220)
Not only are these lines metaphoric, but they are also an example of dramatic irony as the battle at Philippi becomes the nemesis of the forces of Brutus and Cassius. His decision to "ride the tide of affairs" is a fatal mistake for Brutus, who has exercised his free will with error, not properly recognizing the force of fate.