Why does Shakespeare choose to write about a (metaphorical) journey at sea? Think about the time Shakespeare lived in (the era was filled with great sea adventures), and how difficult sea travel was then.

In Sonnet 116, the poet is writing about constancy. He is making metaphorical comparisons between true love, which joins both hearts and minds, and other things that are immutable—fixed and unchanging. The stars in the heavens are fixed in that way.
The ship's voyage metaphor is established with "bark," which is a largely archaic English word derived from the French "barque" (a sailing vessel). Love is also a metaphorical voyage. It depends on guidance from the heavens, or God. Just as the stars guide navigators, keeping them from being "shaken" in "tempests," or storms, so is love like a celestial beacon, keeping the lovers on a steady course. The compass likewise aids in navigation and is a common metaphor for a human perspective or orientation (as in a "moral compass" that keeps people doing the right thing). Here, it also means "scope" or "extent."
https://www.poetryfoundation.org/poems/45106/sonnet-116-let-me-not-to-the-marriage-of-true-minds