“It’s remarkable really,” said Jonathan Beckett, the chief executive of Burgess, a yacht brokerage and advisory firm. “Anyone can go to a website, type in a name and find an owner’s yacht. It’s a little scary.”

For the news media, wealth voyeurs and many of the rich themselves, the ability to track jets and yachts provides welcome transparency and safety benefits. But for the rich who look to their yachts and jets as cocoons of privacy, the mass of digital eyes following them represents an unwelcome intrusion and even a security threat.

“When owners step on their yacht, they want to feel totally safe,” Mr. Beckett said. “A lot of the wealthy prefer to stay under the radar.”

For the most part, yacht- and plane-spotters are more like bird-watchers — hobbyists on the lookout for rare or unusual species in unlikely locales. They loiter on marine docks, boat channels and airport access roads in hopes of spotting and photographing a yacht like Larry Ellison’s 288-foot Musashi or a jet like Paul Allen’s $70 million custom Gulfstream G650.

“Apparently there are a lot of people with nothing to do but hang out at airports and take pictures of planes and their tail numbers,” said Jay Duckson, a private-jet broker and adviser with Central Business Jets.

Yet the growing availability of location data has added a new level of detail to the hobby. Yachts and planes are both required by safety regulations to emit frequent location data. Increasingly, that data is aggregated by websites that let their users freely search for a particular plane or yacht by its name or registration number. The sites also upload up-to-the-minute photos of the vessels or planes that are sent in by amateurs. A quick search on Google can provide the name of a billionaire’s yacht or pictures of a plane.

Most yachts have transponders onboard that send out Automatic Identification System data, which gives the local authorities and other boats their location and navigation headings. Sites like VesselFinder.com and MarineTraffic.com ingest the data and let users type in a yacht’s name and see its most recent coordinates. For a fee, the sites will also send a history of a yacht’s movements going back years. Other sites like SuperYachtFan.com let photographers send in updated photos and owner names for certain yachts.