你改的句子也成立，但不是你想要的那个“最初的写法”，最初的是这样的：That you swim or dive as your body floats on the surface is difficult.（确实头重脚轻，很难受吧？所以用了形式主语it）当然这句如果只要求另一种说法，也可以改成：It is difficult for you to swim or dive as your body floats on the surface....
最初的写法是：That you swim or dive is difficult as your body floats on the surface.为了避免主语太长，谓语太短，我们用形式主语it的结构，写成：It is difficult that you swim or dive as your body floats on the su...
