No boats on the water

An alternative way of describing somebody as smooth brained.

When there are no boats on the water, it is implied that there is no wake and thus the water is calm. This is analogous to the idea that a person with a brain comprised of wrinkles / folds is smarter than one with a brain that is less wrinkly. Thus in this analogy, the brain is the body of water and the wake is the wrinkles / folds (or lack thereof).

Me: Can you believe this kid locked himself out of his house again?
Friend: Definitely no boats on the water for him today.