I killed my tranny in a VW jetta towing a sailboat that weighed 500 lbs. I would not tow a motorboat - an 18 foot will weigh at least 2000 lbs.
You will kill the car.
That being said, a stick shift is typically better for towing - the transmission is not hunting for a gear, and you can choose a lower gear and rev it up to keep from dogging the engine.
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"If you can make black marks on a straight from the time you turn out of a corner until the braking point of the next turn, then you have enough horsepower"
-Mark Donohue
