Written like a classroom assignment, the poem asks the reader to do something impossible: design a container big enough to hold the whole sea. The instructions start with the pleasant things, such as tropical fish, coral, kelp forests, otters and singing dolphins. Then the speaker insists on the rest: great white sharks, red tides, stinging jellyfish. "Leave nothing out."
The tone is firm and a little teasing, like a teacher who knows how tempting it is to keep only the beautiful parts. The repeated "must" ("you must must include") makes that insistence almost urgent. The poem seems to suggest that the sea can't be understood by picking favorites, and the same may be true of anything large and alive.
The ending shifts the mood. After the tsunami comes "the pale blue impossible calms / after the storms have passed." The word "impossible" suggests wonder as well as difficulty. The poem leaves us with both the violence and the stillness, and a sense of awe at how much a single sea contains.