Reported rates of theft and criminal damage on boats in Devon and Cornwall have been running at about 40 incidents a month over the past five years.
Police said that maritime crime was a "big problem".
The figures, obtained by the BBC under the Freedom of Information Act, could be higher, police said, because people did do not always report incidents.
Police advise sailors to treat their boats as they would their cars and make them as secure as possible.
"If you have your outboard stolen, why not tell us?"
Many boats contain expensive electronics, including radio and navigation equipment. Outboard motors can also fetch up to £10,000. Recent rises in fuel costs have also resulted in an increase in thefts of diesel from vessels' fuel tanks.
Robert Piercey has had his boat, the Avocet, broken into twice in Exmouth, Devon.
As well as what he called the "gut-wrenching" effect of theft, one incident could have left him in danger after storage tanks and fuel were stolen.
He said: "The fuel that was left in the engine was enough to start it.
"We were a little late getting out and suddenly the engine stopped. We opened the storage cupboard and found there was no fuel.
"We could have got out as far as the dock entrance. With the tide going, we would have been gone."
National computer
Police said marine crime was a persistent problem.
Pc Matt Helm, of Devon & Cornwall Police, said: "I think it's quite a big problem, and we also have the problem that we're never always sure how big it is because of under-reporting. Not many people want to report it to police.
"But if you have your outboard stolen, why not tell us? You have your serial number, we can out that on our computer and that information will be available nationally. But if you don't report it, we'll never know."
Police advised that expensive equipment should be removed or kept out of sight when a vessel was not in use.
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