My baby turtle afloat??? Help.?

I just received a striped mud turtle in the mail - he is about 1.5 inches long. He is kind of weary of his new enviornment - how can i make him comfortable? Also, when he swims, it seems like he is floating - not lopsided or uneven, but pretty balanced. I have another turtle, and he never floats (but granted, he is an adult). Is something wrong with a baby turtle floating??