The United States Powerboat Show, "the world's largest" goes on display at city dock and harbour Thursday with more than 200 boats and three great tents full of accessories and equipment. The show will continue through Sunday. 10 a.m. to 7 p.m. with a 6 p.m. close on Sunday.

Because at least 125 boats will be in the water and because there is something particularly stimulating about getting on one when the salesman says, "welcome aboard," the sixth annual event should readily satify the yearning boating types have for laying hands on the new and shiny, thumping full hulls and talking nautical.

Those with money as well as those with dreams will want to head for the Hatteras 58 yacht-fisherman. It is a boat that compares favorably with a model apartment of the Watergate. It has wall-to-wall lime-green carpeting a splendid off-white sofa, tasteful drapes, handsome tables, beautiful lamps and wall of expensive teak and mozambique.

Below, there is a lavishly appointed 16 by 10 stateroom master stateroom and something only slightly smaller for two guests. Forward are quarters for crew of two.

The amenities include a dishwasher, laundromat several refrigerators, garbage disposal, stainless steel sinks and stereophonic music piped everywhere.

The craft is pushed by two 325 horsepower diesel engines that use 35 gallons of fuel an hour, although it can over 400 miles at 20 knots before it's time for another feeding.

The craft is priced at $297,000 and one should figure on spending almost $30,000 a year on maintenance, insurance, fuel and a professional captain. Cut out the captain and there's an immediate saving of $20,000.

Something a good deal more modest is being shown by Rogers Morton, a former Secretary of the Interior. His 42-foot vessel has the lines of a lobster boat and is supposed to be able to cope with rugged off-shore weather. It cruises at 12 knots, consumes five gallons of fuel per hour in its single diesel engine and with its 800-mile range can easily make it to Bermuda. Cost: $80,000.

Also capable of the run to Bermuda is the Marine Trader 32 trawler priced at $34,500. In typical trawler fashion, it can't go fast but on the other hand it doesn't need much fuel. Its 80 h.p. diesel engine requires only 2 1/2 gallons per hour to move it along at eight knots. Joe Brenna, who is showing the Taiwan-built boat, says he can't keep up with the orders. "We sell 300-400 a year. In some models buyers have to wait up to four months."

One new feature of this year's show is the Salt Water Sportsman's World and one particularly exotic entry is the AMF Robalo center console fishing machine which, with its 16-foot outriggers jetting off to the side seems to sport a set of giant rabbit ears.

The boat is 25 1/2 feet long and its base cost is $8,900. It can do more than 40 m.p.h., can venture up to 80 miles off shore and, to locate its own position as well as the fish, can carry up to $10,000 worth of electronics. CAPTION: Picture, This Hatteras cruiser, selling for more than $250,000, will be on display in U.S. Powerboat Show. By Bob Burchette - The Washington Post