What an odd question! Notice that it breaks neatly into two parts: the repeated word "who" is the subject of each clause, and "would" rhymes with "could."
Consider the first half: "Who would be a turtle […]?" In the real world, of course, humans don't spontaneously turn into animals, so the question must be posing a fanciful scenario. If a wizard, for example, offered to transform you into a turtle (think of Merlin and Wart in The Sword and the Stone), you might or might not accept the offer.
But then there's the second half of the question: "[…] who could help it?" Our wizard scenario involves a choice, but if a witch got mad at you and turned you into a turtle, you wouldn't have any say about the matter. You couldn't help it.
When you put the two halves of the sentence back together, the question acquires a whole new meaning, something along the lines of, "You'd have to be crazy to choose the life of a turtle unless someone was forcing you to." In fact, the question doesn't even seem like a real question anymore. It's more like one of those pseudo-questions that are actually assertions in disguise, a sneaky way to vent a strong opinion (for example, "You're not really going to wear that outfit, are you?").
At this point, you may be wondering who is asking the peculiarly loaded question in the first line of the poem. Whoever the speaker is, he or she seems to take a dim view of turtles. To find out why, let's read on.
A barely mobile hard roll, a four-oared helmet,
After that testy first line, this line comes as something of a surprise. The speaker uses two quirky metaphors to describe the turtle. At first, the metaphors just seem like a little visual joke, yoking images from wildly contrasting contexts (driving, dining, boating, soldiering) to poke fun at the turtle's shape (dome-shaped like a helmet or a hard roll) and laborious locomotion ("barely mobile […] four-oared").
But these metaphors share a logical connection as well: both convey a whiff of danger. For a hungry teenager, a hard roll is an appealing snack; for a hungry predator, a turtle is dinner. A soldier wears a helmet to defend against enemy attack. The image of a turtle wearing a helmet (or wearing its shell like a helmet) to protect itself from predators is funny, in a cartoonish sort of way, especially with four little oars poking out beneath the helmet. But the image is also a little pitiful.
Notice that the poet connects the two metaphors by using alliteration to link "hard" (which can mean "difficult" as well as "firm") with "helmet." Life is hard, the poem seems to suggest, so don't leave home without your helmet.
She can ill afford the chances she must take In rowing toward the grasses that she eats.
In these lines, the speaker refers to "she," so now we know the turtle is a female. Is that fact significant? Well, the poet didn't have to specify the turtle's gender; animals are typically referred to as "it." So we should probably keep gender in mind as we continue to read the poem.
Lines 3 and 4 seem to confirm the hint of danger we detected in line 2. The turtle's slow journey to find food is a gamble ("chances she must take"); she cannot survive without food, but the search for food also puts her at risk.
These lines hearken back to line 2 in other ways as well. Notice how the sounds in the phrase "a four-oared" create a slant rhyme with "afford" in the previous line (for more on the poet's use of rhyme, check out "Form and Meter" and "Sound Check"). And the word "rowing" in line 4 extends the metaphor of the "four-oared helmet" from line 2.