As flood waters filled the basement of their Minnesota home, Larry and Nancy MacLennan hastily moved their computer to the first floor before evacuating. But the water continued to rise, eventually filling most of the two-story house and submerging the computer for hours.

The machine held thousands of photos–some about 70 years old–and the MacLennans feared those files were lost forever. But when they took the machine to a data recovery firm, engineers had the pictures back in days.

Hard drives may fail when mechanical parts wear out, but they tend to be remarkably resilient in the face of external elements such as water, said Richard M. Smith, an Internet security and privacy consultant at Boston Software Forensics.

''If you look at a hard drive, it's hermetically sealed,'' Smith said. ''In most cases water wouldn't get into the drive itself.''

Computer experts say that hard drives are resistant to water and that data can often be recovered after the are submerged. But recovery can  cost from $400 to  $2,500.That may be good news to boaters, who are increasingly using computers to navigate, maintain a ship's log and perform other vital tasks that result in large amounts of important data. Owners of water- or fire-damaged computers typically assume their files are lost, but computer experts say at least some data can be recovered from virtually any faulty or damaged storage device. And as the computer industry has grown, so has the number of companies doing that restoration work.

''We've done data recovery on a laptop that was dropped from a helicopter, on a laptop that had been submerged in the ocean for a year,'' said Todd Johnson, vice president of operations at Kroll Ontrack Inc., whose engineers helped the MacLennans. ''One time there were even bullet holes in the hard drives.''

Kroll Ontrack is a division of New York-based Kroll Inc., a risk-consulting company. The 20-year-old firm, based in Eden Prairie, Minn., is one of several offering such services, including SalvageData Recovery Lab Inc. in Stamford, Conn., and First Advantage Data Recovery Services in Irving, Texas. The companies charge from $400 to $2,500 for a standard recovery, with the price depending on the amount of data that can be recovered and other factors.

Data-recovery companies use proprietary methods, pulling files into their own environment, where engineers can determine which are salvageable. The recovery process involves digging below the operating system, Johnson said. Data can be salvaged from Windows-based computers and Apple Inc.'s Macs, and even from fully loaded iPods or cell phones. Engineers then ship the files back on CDs, DVDs or on a new hard drive.

Many computer users know they should back up their data, Johnson said, but many keep their backup files so close to their computers that secondary files are destroyed at the same time as the computer. He recommends that backups be kept at a distance.

Of course, data recovery can be a mixed blessing. Sometimes a person disposing of an old computer actually wants the hard drive destroyed to protect private information. So how can one be sure the hard drive is rendered permanently inaccessible?

Some experts suggest running a data-erasing program that repeatedly overwrites information with ones and zeros. Others suggest keeping the hard drive and disposing of the rest of the computer. The most extreme option would be to physically shred the hard drive and dispose of pieces in multiple locations.

Just don't throw it overboard.