Suppose you’ve just updated your good old boat — new power, new electronics, all that — and in the dead of night some creep sneaks up on it and takes away your new $5,000-plus chart plotter.

A lot of us would weep, wail, gnash our teeth and rend our garments. At the end of the fit, we would look around for someone to kill, but the villain, may a thousand curses befall him, would be gone.

That happened to Bruce Marx. The theft, not the tantrum. Marx isn’t the type to lose his head, but: “Suffice it to say, I was outraged by this event.”

If that reads like lawyer talk, it is. And if you don’t know Marx (yes, a lawyer, in Coral Gables), maybe you know who he is: The one who led the successful crusade to save the Bent Range Marker outside the Port of Miami for live bait catchers. The one who led another one against a plan to impose no-take zones on near-shore reefs from St. Lucie to Biscayne National Park.

It was altogether predictable that he would organize the South Florida Marina and Boat Watch Group (m.facebook.com/groups/788946304632934) on Facebook — a medium where, in hindsight, it might have been unwise to post a pic of his new Garmin 7616 XSV on April 7.

He had it until April 14, installed on Harmless Error, his 31-foot Contender. When he went to the dock that morning to wash the boat, the Garmin was gone. The only witness was a video camera — good enough to tell Marx the thief had arrived on a boat and climbed up to Marx’s boat on its lift. Not good enough to get an identifiable shot of him.

It started with Marx posting this announcement: “To everyone who has their boats on the Miami waterfront, beware — there is a gang of thieves making the rounds stripping electronics. You may want to ensure the camera systems at your docks are up and running and that your security personnel is alerted.”

Within a month, his crime watch outfit had more than 1,500 members including the dockmasters of 22 marinas. He thinks it can grow to 5,000.

“After the passage of a few days following the theft and speaking with investigators, I came to learn that marine thefts were on the rise in Miami,” Marx said. “Although I knew my GPS was long gone, I wanted to commence an effort to organize a marine community watch to prevent others from sharing my experience.”

Another Facebook post asked boating friends whose electronics had been stolen to email him with details. He would give the info to the Miami Police burglary squad.

Marx’s friend Scott Baxter soon joined him to put together the watch group, which jumped on plane and sped into existence on April 27 — just 13 days after the ripoff that inspired it.

Not everyone could have done it that quickly. Marx used a humongous email list of fishing and boating contacts, accumulated during his prior crusades. Knowing his ability to get things done, they swarmed to leap aboard this one. He’s getting local police and Fish & Wildlife enforcement involved as well.

The idea is simple — creating a place where boat, accessory and fishing tackle theft information can be spread quickly. Though the name implies an emphasis on docked boats, Marx said it’s intended also for trailered boats, motors and fishing tackle.

“With the immediacy that social media provide, we want everyone to be able to instantly share boat-related theft information,” Marx said. “Not only will law enforcement be in a position to assist, but the boating community will be able to assist, as well.

“There will come a time when someone sees a boat being trailered down the road, on the way to the chop shop, that they saw posted on the SFMBWG page. This is the type of result we are looking to achieve. The more eyes and ears we have out there sharing information, the better position we will be in to catch the bad guys.”

We asked Marx by Facebook message if he’s replaced the stolen chart plotter yet — perhaps with one that’s just as good but not as popular as Garmin.

“Haven’t replaced it yet,” he answered. “Will be getting the same one. Beefing up security first.”