Your poem is almost perfect and very nicely written! Here is a corrected version with slight improvements for better grammar and flow:
With a shell, it crawls slowly on the sand.
Its flippers are made for the sea, not the land.
The original lines are grammatically correct. However, if you want, we could make it even smoother by changing it to:
With a shell, it slowly crawls on the sand.
Its flippers are made for the sea, not for the land.
Both versions are good, but adding "slowly" as an adverb right before the verb "crawls" makes the sentence sound a bit more natural. Also, repeating "for" in the second line clarifies the contrast.