The blooming of the pussy willows on King Street signaled the de-icing of the Potomac - that and the cranking of the huge engines on the good ship Dandy. She loads up, says goodbye to care and woe, hoists anchor and casts off from Alexandria for a new season of starlight cruises, high life on the water that includes good food, good drink, dancing to a live band and just dallying as you and nearly 200 fellow passengers lazily circuit the Potomac.

And if you've played your cards right, you're with good company. If you have anything to celebrate, or hope to have, you've come to the right place. (But bring money! Even romance costs these days. Dinner and cruise package starts at $12.50. Drinks and tip are separate. Dessert adds about $1 per person, and parking is $1 per car.)

Two main thrills on the water, as any cruiser knows, are the excitement of feeling the engines start to pulsate under you, signaling that something adventurous is bound to happen, and the unadulterated romatic fantasy of stepping into the cozy, intimate "main deck" restaurant that looks and feels so much like one on an ocean liner. It's the essence of romance, capturing that mood that's always been a part of the water's lure - all by candlelight.

The Dandy's owner, Donald S. Davis, and the restaurant staff and ship's crew, have gone to great lengths to keep the mood rolling throughout the three-hour ride. The glassed-in walls around you offer not only temperature control year-round, but a unique view of many of Washington's favorite monuments. You glide slowly past the Washington and Jefferson memorials, their lights adding to the mystique of the changed perspective, past the Kennedy Center and into Georgetown, up to Key Bridge.

"Definitely a new way to see the city you thought you knew from every angle," says Davis, who set the ship afloat in the area for the first time in June of '75.

He got the idea while living on his own boat in Paris, where he was attached to the embassy. "I realized that the best view of the town was from the Seine. Eighty percent of the beautiful natural elements and monuments were more visible from the water. Washington was exactly the same."

So, he bought the ship, a 100-ton, 100-foot vessel designed to do just what it does - function as the "only luxury reataurant-cruise ship now operating on the Eastern seaboard, to my knowledge."

And, despite his lack of serious advertising efforts, Washingtonians and visitors who've learned about Dandy by word of mouth have bought it, too. During last season's cruising schedule (mid-March to the end of October, when the river often was too surly to be managed) the Dandy drew 55,000 people.

And so far this season - which officially opened only in Mid-March - advance reservations stand at 10,000. According to ticket agents, people call as much as six months ahead to book the Fourth of July. Tuesday through Thursday evenings are easier than Friday through Sunday, but reservations are a necessity anytime.

Aside from the purely romantic elements, there are the food (good) and a two-man group that manages to sound fully orchestrated - and it's not just the ship's engines dulling your senses. While you peruse the menu, shortly after the 7:30 cast-off, the "Vern Coleman Orchestra" puts everything right with a loud "Anchors Aweigh." Then they drop down a decibel or two, as you swing out of Alexandria's harbor and start the arc of upriver, and slide into BeeGee's material. Or whatever suits. Waiters are polite and attentive without hovering. They smile a lot, and ask you to call them by their first name.

The menu is imaginative - "Seafood Creole" ($12.60) or Bengal Beef Curry with chipped nuts and coconut ($13.75), which they warn you is hot - but lovers of spicy food will be disappointed. The Captain's Standing Prime Rib is a big draw at $16. Ship's Salad, which comes with the meal, is quite good.

A thorough attempt has been made to supply voyages with a range of wines, from dry California white to French and Italian roses and whites. Champagne is available, too, from $16.50. (That comes in handy when there are weddings - there have been five aboard, so far - and company parties, where someone is always having a birthday.)

After dinner, and drinks, and coffee, get up and stroll the room, admire the scenery from the other side of the boat.And dance. There's a floor that's bigger than one at Tramp's. New this season, also, are weekend floor shows for late-nighters after the cruise docks: one at 11 and another at 12:15.

Pop the question or champagne cork marking the indelible years comtemporary, marking the end of the years, or whatever. In an evening's time you can live a lifetime of adventure - or start one. Not bad, for about $16 to $22 a head.