Just about everyone knows that drinking and driving don't mix. You can double that for drinking and boating.

Unfortunately, a surprising number of people who wouldn't consider driving a car while intoxicated, apparently don't think twice about boating after imbibing too much.

Since 1980 there have been 75 fatalities and $2 million in property damage in alcohol-related accidents on Maryland waters alone. What's more, the Maryland Department of Natural Resources Marine Police reports that alcohol was involved to one degree or another in 75 percent of the more than 600 accidents in that same period.

While people consider their cars as transportation, says Marine Police Sgt. Edward Waddell, they tend to view their boats as recreational vehicles. Along with the life preservers, water skis, fishing and crabbing gear, a six-pack or more of beer is considered standard equipment. "It's a part of their fun relationship with the water."

Boating enthusiast and Maryland University Extension Marine agent John Schwartz warns that the danger isn't just a matter of obvious over-indulging: "The social drinker who can tolerate several drinks indoors may become drowsy and befuddled drinking in the summer sun." And the result may be more than a crunched bow or stern: 47 percent of adult drowning victims show alcohol in their blood.

The effects of alcohol, say Schwartz and Waddell, can be intensified by wave action and stress and fatigue from several hours on the water.

Both men regard a recently imposed Maryland law as a positive step in discouraging alcohol or drug abuse by boaters. The law, which became effective July 1, imposes a first-time penalty of up to three months in jail and/or a $500 fine for operating a boat while under the influence of alcohol or drugs. Second-time offenders face up to one year in jail and/or a $1,000 fine.

"I hope," says Waddell, "that the law will help create an awareness in people, encourage them to have second thoughts about how much alcohol they have on board or consume themselves." Says Schwartz: "The law isn't out to nail the casual drinker. It's aimed at those people who are intoxicated and operating a potentially dangerous vehicle."

D.C. and Virginia both have laws calling for penalties of up to 1 year in jail and/or a $1,000 fine for negligent operation of a boat, considered a misdemeanor.

"Whenever you have increased congestion," says Schwartz, "and we have more boats out on the Bay every year, the chance of accidents is increased. Add alcohol to the formula and the risk goes even higher." Approximate current boat registration: D.C., 4,000; Maryland, 135,000; Virginia, 140,000.

Maryland Natural Resources Police Capt. Franklin Wood has another way of putting it: "If the impact of a car crash doesn't kill you, you have a good chance of surviving. Because of the risk of drowning, in a boating accident you can sustain only minor injuries and still wind up dead."

* Going sky-high: Chevrolet's high-tech 1984 Corvettes are selling well despite their $25,000-$27,000 price tag. What buyers are all heated up about includes an electronic instrument cluster with microcomputer-controlled liquid-crystal displays. Speedometer and engine rpm data are displayed in both digital (numerical) and analog (graphic) form.

* The Council of Better Business Bureaus has a couple of informative brochures that should be of interest to anyone contemplating the purchase of a car: Tips on Buying a New Car and Tips on Buying a Used Car.

For vacation travel, two other pamphlets, Car Care on the Road and Tips on Car Repair, might be helpful.

They're all free if you go by the BBB offices, or they can be purchased by mail, 25 cents plus a self-addressed, stamped business-size envelope, from BBB, 1515 Wilson Blvd., Arlington, Va. 22209.

The Consumer Information Center, Pueblo, Colo. 81009, also has, for free: Common Sense in Buying a New Car and Common Sense in Buying a Used Car.

* Something new in used cars: Businessman Steve O'Hara has eight dealerships in central Florida specializing in quality-guaranteed used cars. They're being sold under the brand-name Detroit II. O'Hara says he hopes to have several thousand outlets across the country by 1987.

* Something old in used cars: More than half of all Rolls-Royces built since 1904 are still on the road.

* Something new in any car: A window shade for automobile windshields and windows. Inventor Norman F. Landon claims that the Karinna Sun Shade (attached with suction cups) not only blocks direct sunlight, but also helps protect your car's dashboard, seats, steering wheel and speakers from overheating, cracking or fading. The white laminated plastic shades are $9.95 from Box 642, Hollywood, Fla. 33022.