Poem found here: "A Man Young and Old" III. The Mermaid" by William Butler Yeats
This poem is like an Aesop fable even with the didactic message at the end. And even though this poems intent is a bit obvious after reading the poem, it doesn't mean that this poem cannot bear interest after each read.
"A mermaid found a swimming lad, / Picked him for her own, / Pressed her body to his body." At face value, this beginning has the mermaid take the lad as her own. But note, there is no mention of love, just ownership. Also, when her body presses up against his there's a sense of fusion or rather something akin to Adam and Eve, Eve and Adam.
"Laughed; and plunging down / Forgot in cruel happiness / That even lovers drown." It's not the didactic tale that keeps me reading this poem, it's how the poem is shaped syntactically. The stand alone verb of "Laughed" reinforced the past tense with a more visual verb (pressed is visual too, but not as "human" as laughed). And then when they plunge down, they forget with "cruel happiness" here the adjective/noun combination which brings a sense of irony foreshadows the didactic end.
"That even lovers drown" I guess this could be a play on the inability of looking at death when in love or how the twisting of emotions that makes up love makes us blind. The poem is didactic, but what is the lesson learned?