They weren't targeting any of the new boats for sale lined up in the back. As surveillance video clearly shows, they were after the Yamaha engines.
"These thieves were so sophisticated that they walked past engines that were sitting in the boxes that they could have just loaded up," May said. "They had our forklift running. And they took specific types of engines that are more valuable."
Incredibly, May has been hit by engine-stealing thieves not once, but four times. May owns six boat dealerships throughout Texas and estimates the 41 Yamaha engines he's lost were worth more than $700,000.
The video from the theft at his Houston facility shows the suspects entering the property early on a Sunday in July. They had cut a hole in the fence, and then pulled in with a pickup truck and flatbed trailer.
"Some of these engines actually belonged to customers," May said. "So not only did we lose product, we angered some customers and had to replace their engines for them."
May is not alone.
Industry experts and law enforcement officials told CNBC that boat engine theft —specifically of Yamaha engines — is on the rise.
While boat theft nationally increased 1 percent in 2016, there are no national figures for boat engine thefts, according to the National Insurance Crime Bureau.
Florida, which ranks as the top state for boat thieves, does keep track of engine thefts. A CNBC analysis of records compiled by the Florida Fish and Wildlife Conservation Division shows a total of 811 engine thefts in 2016 compared with 643 in 2015. About half the stolen engines were Yamahas.
"I will you tell that outboard theft from marinas, outboard theft from dealers has gone up dramatically," said Dan Rutherford, director of claims and risk management for Maritime Program Group. "In my 35-year career, I've never seen as many outboards and as many units being stolen from as many marinas."
Rutherford, who suspects a number of the engines are being shipped overseas, said the thieves typically break through a fence with a stolen truck.
"And there are three guys usually. And they're just lifting the engines and putting them on the truck beds," he said.
The recovery rate for stolen boats in Florida last year was 36 percent, according to the National Insurance Crime Bureau. In California, the second most ranked state for stolen boats, it's only 55 percent.
"It's a great crime of opportunity," Rutherford said. "It's a difficult crime to solve. The boats disappear."
Miami-Dade Police Sgt. James Barrett said boats and boat engines, unlike cars, are more difficult to track once they are stolen.
"The data is not out there mainly because with boats there's a lot of different manufacturers," Barrett said. "And they come and go. And then once they go out of business, it's hard to get those records. We do have some boat manufacturers that are very cooperative, that do keep good records, and they'll give us a lot of history on that boat. But on motors, it's just a serial number."
Officials from the Coast Guard and local law enforcement told CNBC they have not tracked large number of the stolen engines overseas, at least in recent years. Instead, thieves typically try to replace the engine's sticker with another one, making it appear the stolen engine is legitimate.
Many boats, and their engines, are often easy targets for thieves whether it's at a marina or in someone's backyard. Some 5,115 watercraft were stolen last year in Florida, according to the National Insurance Crime Bureau.
The Miami area is number one in the country for stolen boats, followed by Tampa, Fort Lauderdale, Orlando and Tavares.
"Unfortunately, with the amount of waterways that we have in Miami-Dade County, it is a target-rich environment," said Miami-Dade Police Officer Miguel Espinosa. And sad to say, it is very easy [for] someone to try to steal a vessel. Or engines, especially engines."
To deter theft, Yamaha Marine Group told CNBC, boat owners can purchase the Yamaha Customer Outboard Protection, or Y-Cop. The system disables the fuel-injection systems when its owner touches a key fob.
"Y-COP comes standard in all Yamaha Helm Master systems, and is available for use with any command-link compatible Yamaha outboard of 25 horsepower or greater," said Yamaha Marine Group spokesman Austin Roebuck. "Y-COP is an easy-to-install and simple-to-use theft deterrent system that is useful for preventing theft and identifying stolen outboards. If a consumer ever experiences a case of outboard theft, we recommend that they secure a police report, and contact their local dealer or Yamaha Marine directly at 1-800-894-1626."
The hurricanes in Houston and Miami temporarily slowed down the thefts, but no one expects that to stop the thieves.
May said he's done everything possible to deter engine thefts. "Your first instinct is to get mad and get revenge," May said. "But ultimately, you just have to take care of your business."