Quote Originally Posted by Augury
Judging from how the commander phrased his trap and the last scene of the crab climbing up the rocky shore, my guess is that there's some sort of high tide or low tide that prevents ships from leaving. So the commander could just be looking at the water level and timing.
Yeah, that's what I figured out also, and when he says something about the final trap at 9, you see water hitting the edge of the rocks, and being a set time, that's all I can figure.

The Strawhats must just escape through the tide, and then it's the end of the filler, sweet.