There are actually two sentences here. Because it is not indicated to which sentence you are referring, I will look at both.
For half an hour the glass-bottom boat drifted. CORRECT
Pitching and rolling above the reef in four- to six-foot waves. FRAGMENT
The second sentence is a fragment because there is no subject within the sentence. Even though it can be determined that what is being pitching and rolling is the boat, sentences should always be considered independently of each other. Thus, when look at the second sentence we are not told what is doing the action. As such, it is a fragment.
