Margaret Atwood's poem "Siren Song" is a modern take on an ancient Greek myth. An alluring female (appearing at least partially human) sings an irresistable song that leads men to their deaths. The opening references to drowned soldiers in Atwood's poem clearly draws on this Greek myth, in which sirens lure men into the water to drown them. All this, I would say, is the poem's subject.
The poem's theme is a little trickier to pin down. Perhaps the theme is that some things are so irresistable that even when we know they're not going to do us any good, we still do them. In Atwood's poem, the speaker is the siren, and the listener (the "you"), even though forewarned, ends up drowning like all the rest of the victims. The theme might be applied to gender stereotypes; we know that stereotypes aren't always useful in the real world, but many of us still fall for them over and over.
In terms of form, this poem is definitely open form. There are no regular line lengths, end rhymes, or stanzas. The frequent use of enjambment (such as the surprising break between lines 1-2) is a common feature in open form poetry. The break is surprising in lines 1-2 because it splits apart the subject ("everyone") and the verb ("would like"). A similar break is found in "This song / is a cry for help." Usually, in traditional and closed form poetry, you'll see the subject and verb together, not split apart.
The first link below leads to a discussion of the mythological figure of the siren. The second link should take you to two previous discussions of this poem.