Symbolism, Imagery, Allegory
King Henry's over-the-top boat tour highlights his absurd level of political stagecraft. It's like every presidential debate of all time, rolled into one.
We can tell that something's up from the moment that Cromwell first describes the tour to Chapuys. Although it's clearly a big, expensive publicity stunt, Cromwell frames it as the intrepid King leading a ship as its captain. Check it out:
CROMWELL: [...] You may not know that the King himself will guide her down the river [...] He will have assistance, of course, but he himself will be her pilot. (1.406)
It's kind of like when you see a president driving a tank or jet fighter in an attempt to look rugged and tough—it's totally inauthentic, and it never looks right. For his part, Henry seems to wholly believe his own hype, bragging about the mud on his shoes like he's a real wild man:
HENRY: No ceremony, Thomas! No ceremony! (They rise) A passing fancy—I happened to be on the river. (Holds out a shoe, proudly) Look, mud. (1.504)
Henry even tries to wrestle Norfolk in a further attempt to prove his manliness, despite the fact that everyone knows Norfolk could beat anyone in wrestling. In other words, Henry at this point is mad delusional, a fact that is further emphasized by Henry continually trying to frame his visit to More as a personal one, despite the fact that we know this is a politically focused stop on a pre-planned publicity tour.
All in all, King Henry VIII's little boat tour unearths the hypocrisy of the British political order—a hypocrisy that ultimately proves to be Thomas More's undoing.