The correct answer is 13 people.
In the first trip to the island, 5 people get on the boat. Now, someone has to drive back to pick them up. Since there's already 1 person in there, he can only pick up 4 more. That's 9 thus far. He goes back and only has the last chance to pick up another 4 because he's already spent 18 minutes rescuing as many as possible. So, by the time the driver makes it to the island, the ship will have already sank and the last two people would be food to the man-eating sharks.
A moment of silence for the two that didn't make it, please.
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So, Stitch and lbt/cty_lover got it right!

Scores thus far:
Stitch: 40 points (10 Hint Points)
Flathead770: 0 points (10 Hint Points)
Mornai: 50 points (12 Hint Points)
Clawandfang: 200 points (14 Hint Points)
LBTFan13: 0 points (10 Hint Points)
Tails_155: 80 points (12 Hint Points)
lbt/cty_lover: 125 points (12 Hint Points)
Mumbling: 50 points (11 Hint Points)
action9000: 0 points (10 Hint Points)
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Okay Stitch, it's your turn.
(Ha! Finally stumped ya, Clawandfang!

:)