What metaphor does Brutus use to convince Cassius to fight in Philippi in act 4?

We need to look at act 4, scene 3 to find this. Brutus's famous speech to encourage Cassius to go to war begins at line 215. He uses several metaphors here as persuasive techniques. First, he says "our legions are brim-ful," suggesting that they are "brimming," like a bucket of water, with strength, energy, and men; he says "our cause is ripe." The cause is here compared to a fruit which has reached its perfect stage of development and is ready to fulfill its purpose (that is, to be plucked and eaten).
The next metaphor we see Brutus use is an extended or epic metaphor, which continues over several lines. This begins with the statement that "there is a tide in the affairs of men." Brutus says that he and Cassius are currently "afloat" on this sea of change and that their fortunes could be swept in either direction if they do not act appropriately. This sea, "taken at the flood, leads on to fortune." Brutus is saying that Cassius must act now, because if this moment is not seized, he will instead be "bound in shallows and in miseries."
Brutus is successful in his attempt to encourage Cassius with this vivid imagery, and Cassius agrees that he will take his army on to Philippi.