and maggie discovered a shell that sang so sweetly she couldn't remember her troubles,and
In the second stanza, we get to know maggie. It looks like she found a neat seashell that sings. You've probably found a shell like that before at the beach. Typically, it looks like a conch or scallop shell. When you hold it up to your ear, you can hear the hypnotizing hum of the sea in there. And, hey, that hum fits nicely with the hypnotizing dactyl beats we heard in line 1.
And actually, that's kind of the point here with the sonic imagery of the singing shell. Maggie is so enraptured by the magical sounds of the sea that she can't remember any of her troubles. It's like the ocean has this hypnotizing effect that she can't help but feel soothed by. (We can relate.)
That's not all of the sounds that are going on here, either. We have more alliteration, with the repeated S sound in"sang," "so," and "sweetly." That seems like an appropriate choice. All those S's seem to mimic the sea's waves.
Rhyme-wise, we'd say that "sang" and "and" are close enough to count as end rhymes. They aren't perfect rhymes, but they add to the speaker's playful, airy tone in describing this good-times beach trip. (Hit up "Form and Meter" and "Sound Check" for more on how this poem is put together, Shmoopers.)
Finally, we see some more squishing in line 4 too (not a typo). "And" is squeezed into "troubles," which again makes that line sound super-casual and story-like, and makes the speaker sound a bit like a kid who needs to rush to get everything out that he needs to say (we're just assuming the speaker's a he at this point). Kids do that a lot, especially with the word "and." And then I went to the beach, and then there was a sweet shell, and then this crazy crab came along, and, and, and…