Symbolism, Imagery, Allegory
You've probably noticed that there are frequent references to swelling bodies of water in the play – especially flooding rivers and rising ocean tides. Come to think of it, most of these references are related to rebellion and disorder, which makes a whole lot of sense, given that rebellions and bodies of water can be dangerous and destructive when they "rise" up and breach set boundaries.
So, when one of the rebel leaders (Northumberland), shouts "Now let not nature's hand / Keep the wild flood confined! Let order die!" (1.1.12), he's calling for a great surge of rebels to rise up against King Henry IV, an authority figure that's supposed to keep the rabble in check. Northumberland is also furious because he's just learned that his son has been killed by Prince Hal so, we can also think of a "wild flood" as an apt metaphor for Northumberland's rage (expressed here in his powerful language), which is literally spilling out of him at this moment.
Another significant reference to swelling waters occurs when Hal promises his brothers and the Lord Chief Justice that the wild days of his youth are a thing of the past.
The tide of blood in me
Hath proudly flow'd in vanity till now:
Now doth it turn and ebb back to the sea,
Where it shall mingle with [the sea]
And flow henceforth in formal majesty. (5.2.4)
Here, Hal associates his rebellious past with a flowing "tide." This is an interesting association because it seems to align his unruly behavior (carousing with the commoners, stealing, thumbing his nose at his father's authority, etc.) with Northumberland's civil rebellion. This isn't so surprising, given that Shakespeare makes so many parallels between Hal's unruliness and civil rebellion throughout both parts of Henry IV. The difference between Hal and Northumberland, however, is that Hal also says here that he's prepared to leave his riotous past behind him (the "tide" will "ebb back to the sea"), while Northumberland makes no such comparable statement.
So, we've clearly established that floods and tides are associated with rebellion. The thing about floods and tides is that they're also a very natural phenomenon. Does this mean that Hal's rebellion against his father and the rebels' civil revolt is natural and/or to be expected? If so, does the play also suggest that after a flood/rebellion, it's also only natural that the waters recede/rebels go back to being obedient subjects? What do you think?