Booby. Trapped.

UPDATE: Bob Shirley has an excellent site devoted to the Vietnam Swift Boats at Patrol Craft Fast. In his comment he cites to a link concerning a booby trap and the sinking of a boat that investigated the trap. If the link in the comment doesn't work for you, hit the link above, then go to "An Thoi" at the top of Bob's page and then scroll down to where it says "View the story of the Loss of PCF-4" -- it's worth a visit and is a part of history unknown (or unremembered, since I have read the Cutler book) to me before Bob commented.

Or maybe my recreation of the link will work.