LIGHTHOUSES ARE such sturdy, friendly landmarks that one wishes they were immune to all the ravages of weather, time and man. They're not - as shown by the fate of the priceless, 96-year-old crystal lens on the Sandy Point Shaosl lighthouse, which was smashed by an intruder recently. And senseless vandalism is only one of the threats to the surviving lighthouses on Chesapeake Bay and elsewhere. As navigational methods change, many old beacons have become secondary aids to mariners. The Coast Guard has abandoned some and automated many - a step that may save as much as $170,000 per lighthouse per year, but leaves the unmanned towers more vulnerable to vandalism and decay.

Not all the lights are going out. Some of the nation's most important lighthouses have been maintained by the Coast Guard. More than 30, including the Cape Hatteras light and the ones around the Golden Gate, are in the National Park Service's careful custody. A few of the Chesapeake Bay beacons, including those at Cove Point and Thomas Point, are designated as historic structures and will be preserved.

But what about the rest? The Coast Guard is understandably torn. Refurbishing and manning offshore structures such as the Sandy Point light would cost more than their navigational role can justify. Still, however ungainly and outdated some of the lights may be, abandoning them means giving up traditions - and the Coast Guard is reluctant to do that.

The service is now exploring several possibilities. One that deserves to be tried is leasing some lighthouses to civilian occupants or caretakers who could maintain the structures as well as the lights. Others no longer needed for navigation could be declared surplus and transferred free to non-federal park agencies or preservation groups. For any of these routes to work, though, people who like the lighthouses will have to come to their aid. And public investment is appropriate, because of the beacons have come to mark more than locations on a costal map. They still reflect the spirit expressed in a Lighthouse Board report in 1868: "Nothing indicates t, e liberality, prosper ity or intelligence of a nation more clearly than the facilities which it affords for the safe approach of the mariner to its shores."