Corrected entry: It doesn't make any sense that Portman places a bomb in the boat as his only target Rachel was the least likely to take a round on the wintry lake.
Correction: He put the bomb in the boat to hurt Fletcher, as this would hurt Rachel.
Continuity mistake: At the Academy Awards scene, when Frank shoots Portman the second time, Portman receives an (entry or exit) wound on his temple. However, the way he was standing after being shot in the chest the bullet could only have hit him from the front. (01:53:10)
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Suggested correction: The second shot hits the camera. The wound on the side of his head is debris from the camera.
Correction: I actually believe the bomb was intended to kill Frank so that Rachel would be an easier target. There is a scene of Frank teaching Fletcher how to use the boat. Portman probably assumed that Frank would take Fletcher out on a ride.
This is exactly what I felt, too. I don't think the goal was to specifically kill Fletcher. He would've been an incidental casualty.