Or on the rainbow of the salt sand-wave, (16)
What could be more transiently beautiful than a rainbow? Especially the rainbows that appear in the ocean spray of a wave on the beach? They just hover in the air for an instant before the spray disperses and falls. That's the kind of transient beauty that the speaker wants us to think about.
Or if thy mistress some rich anger shows, Emprison her soft hand, and let her rave, And feed deep, deep upon her peerless eyes. (18-20)
Even anger is a fleeting, transient emotion, so you should allow yourself to experience it fully—even if it means sitting back and letting your mistress "rave." According to our speaker, all emotions should be experienced to the fullest possible extent.
She dwells with Beauty—Beauty that must die; (21)
The "she" in this line refers to the "mistress" of the previous stanza—the speaker reminds us that her beauty won't last forever. But the "she" could also refer to a personification of Melancholy. Even melancholy won't last forever, which is why you should be alert and appreciate it as you feel it.