I don't understand the boat explosion
When Frank gets Fletcher out of the boat, and they're on the little pier, the boat explodes. I assume that it was some kind of remote detonation, like the boat had been rigged with explosives, but I don't know that this is ever explained.
After the boat explodes, everyone is in a panic at the cabin, packing up all their things and getting ready to leave.
Why would the boat be rigged to explode? Did the killer assume that they might use it and so rigged some kind of timed device? If the killer was watching, why would he explode the boat when Rachel wasn't in it, let alone anyone else?
I'm sure I am completely missing something, but I don't know what it is.