Ok, I worked the crime and fire beat in Houston
for almost five years. You guys are seriously paranoid. Call your local law enforcement agency, tell them your situation, and they'll be happy to help you if they have the time and resources. If they don't have the resources to send a dog out, at least it's on record
that you called about it should something ever turn up later.
We had this exact issue occur at one of the high schools in the area I was covering. A kid buys a car. A week later in the school
parking lot, the drug dog alerts on the car. They found a spent joint matted into the carpet under the floor mat. Because the schools had a zero-tolerance policy, the student was expelled. However, he was an honor student and it was finally proven that the joint pre-dated the sale
of the car.
Personally, I would just do a thorough search and cleaning
of the boat, but as forgetful as stoners can be, they never seem to forget their weed.