Revealing mistake: McGarrett's office is on the 2nd floor. Yet the photo backdrops outside his windows in this episode show us a ground-level view of grass, streets and a building. This boo-boo vanishes in subsequent episodes, replaced with the more likely views of balconies, palm trees and blue sky.
Continuity mistake: There's much too abrupt a transition between Reese falling into the water and his body suddenly being back on the dock. As there's no time lapse implied at all, he seems to reappear on shore by magic. At least a brief shot of the body being fished out of the water might have helped.