(1) Sea Level or (7) Snow Line!
This poem is as tough as you want to make it. If you take the three-line poem at face value, it doesn't seem so difficult: the language is clear and straightforward, the tone is conversational, and you could basically read the entire thing at a glance. You could also still get a sense of the poem's wittiness and be moved by its commitment to and faith in poetry.
On the other hand, if you begin trying to define precisely what Moore means by the "genuine" in poetry, or contemplating why she includes a longer version of the poem in her endnotes and if this means we must read the two versions together, suddenly the poem starts to raise complex theoretical and methodological issues and escalates up the toughness scale.