Upon listing the 37-foot boat, docked in Key West, Florida, on a rental platform called Boatsetter, he booked 11 trips in a month.
Today, Mancini's two boats bring in an average of $38,800 in revenue per month, according to documents reviewed by CNBC Make It.
That's off only 30 minutes of work per day, spent managing bookings and making sure the boats' captains — who are hired and paid by individual renters — are maintaining his watercrafts properly, Mancini says.
Instead of using the money to pay down the $550,000 in loans he took out to buy the boats, Mancini says he plans to funnel it toward more watercraft purchases and other real estate opportunities.
Here's how Mancini used his sales experience to launch his "mostly passive" six-figure boat business, and what waters he plans to charter next.