We lose a lot of crewman when the Green Hornet goes down, but watching Mac's slow decline into madness might be the most difficult death to watch.
Along with Louie and Phil, Mac survives the crash into the Pacific, but it's all downhill from there. First, he eats all the survival chocolate, eliminating days' worth of rations in a few seconds. Even though Louie understands that Mac did this in a moment of panic, he's still not happy about it.
In a life or death situation, it's important that everyone keep their cool, and Mac does not keep his cool. He starts screaming "We're going to die!" (3.12.17), and although Mac stands up (literally) at one point and whacks a shark with an oar—saving Louie—he soon returns to his near vegetative state. Eventually he dies, and Louie and Phil wrap him in part of the raft and drop him into the water. "Mac sank away. The sharks let him be" (3.16.31). And that's the end of Mac.