The poem you wrote is mostly correct and easy to understand. Here is your original poem with a small suggestion to make it even clearer:
With a shell, it slowly crawls on the sand.
Its flippers are made for the sea, not for the land.
The grammar is good! The poem paints a nice picture of a sea creature (like a turtle) that moves slowly on the sand and has flippers meant for swimming in the sea instead of walking on land.
If you want, you could say "With its shell, it slowly crawls on the sand" to make it clearer that the shell belongs to the creature. But your version is perfectly understandable and grammatically correct too.
Great job writing your poem!