IF THIS IS Tuesday, it must be Grenada." Capt. Peter Jackson of Cunard's Countess was commenting on the ship's regular weekly itinerary of six ports in seven days.

Grenada has perhaps the most picturesque harbor around. It's a sight that high-flying jet passengers never see. Even strolling around it does not have the impact of sailing into this Caribbean port. Not all Caribbean ports are as enjoyable to enter but, if you are interested in sampling the Caribbean, a cruise is perhaps the best way to do it.

We had selected the Countess for its itinerary. From San Juan in Puerto Rico, the ship calls on Caracas, Venezuela; Grenada, Barbados, St. Lucia and St. Thomas. Its sister ship, the Princess, also follows a seven-day, six-port schedule, calling on Tortola, St. Maarten, Antigua, Martinque and St. Thomas.

Depending on your interest, a Caribbean cruise can be as simple as a two-port schedule or six or more ports, depending on the length of the cruise.

If you choose to leave from San Juan, you can also select from the Costa Cruise Lines Carla C, which leaves every Saturday for Curacao, Caracas, Grenada, Martinique, St. Thomas; or the World Renaissance that visits St. Lucia, Barbados, Guadeloupe, Antigua and St. Thomas.

Many ships leave from Miami. The Boheme visits Puerto Plata, St. Thomas, San Juan and Cap Haitien. The Carnivale visits Samana in the Dominican Republic, San Juan and St. Thomas. The Festivale calls on Nassau, San Juan and St. Thomas. The Mardi Gras' ports are Nassau, San Juan and St. Croix.

The ships of the Royal Caribbean Line, the Song of Norway and Sun Viking, take seven-day cruises that alternate ports of call each week, so it's possible to stay aboard ship and make it a 14-day vacation.

From Fort Lauderdale, Sitmar Cruises offer a variety of 7- to 14-day cruises through the Caribbean.

Norwegian Caribbean Lines has the Norway, the world's largest cruise ship. It only calls on a small island in the Bahamas and St. Thomas. In this case, the ship itself is the destination. The line's others ships, the Southward, Skyward and Starward, also offer Caribbean ports.

Home Lines will be offering its seven-day cruises out of New York aboard the Oceanic to Nassau. Some sailings will also carry on Bermuda.

The cruise opportunities are many. Selection is difficult. After all, ships, like people, have personalities of their own. These personalities develop from the size of the ship, nationality of the crew (all-Italian or many nationalities), length of cruise and the number of ports visited.

On some cruises the ship becomes the destination. Your vacation becomes centered around shipboard activities, the crew personalities and passengers. Aboard the Countess, life is different. Being away from the ship for great stretches of the day creates a distance between you and ship life. But after a day of touring ashore, it's good to know you'll be back among acquaintances and in an atmosphere that's relaxing.

On other ships with only a couple of ports of call there is a mix of shipboard life its relaxing pace and the opportunity to sample island life.

While we enjoyed the traditional shipboard activities of dining, parties, shows, swimming and lounging, the daily schedule aboard the Countess was still determined by the island we were to visit. Some days it meant rising early to make the most of an 8 a.m. port arrival; other days the pace was slower. At Grenada, the ship docks at 2 p.m. for an afternoon visit to the St. George area.

Ample time is allowed for shore visits but we found that pre-planning pays off. Armed with a copy of "Fielding's Caribbean Guide," we could plan what we wanted to see at each port.

There's hardly a ship afloat that doesn't offer shore excursions. Cunard is no exception. But with our guidebook we found we could hire a taxi, see the attractions we wanted at our own pace and still have time left for a relaxing swim at one of the island beaches. And, we could do this at or below the ship's excursion price.

Since Caracas, Venezuela, is a large, busy and confusing city, we found the ship's tour at $32 a person, including lunch, a good value. However, on Barbados we opted for our own tour by taxi. Outlining our plans with the driver, we managed to include shopping, touring, lunch and an ocean swim in our day there.

In planning a Caribbean tour, it's good to keep in mind your purpose: a lot of sightseeing or a relaxing vacation. Cruises offer good value: Just about all your costs are included, with the exception of bar beverages, tipping, and shore excursions. However, on a multi-island tour, those excursions can add up. Prices range in most cases from $10 to $32 per person and may not include the lunch that would be included in your cruise price.

Combining island sampling with a cruise is both enjoyable and, at this time of year, a good value. Some lines are offering discounts and just about all lines have a low-cost package for a third or fourth person sharing a cabin. Special rates for children, along with planned shipboard activities for them, make a summer cruise a good family option.

An added value comes with an air-sea vacation. Cunard offers a couple of unusual vacations. You can combine a week's stay at a resort on St. Lucia or Barbados with a cruise, or leave one of their ships and join the other for a two-week, 10-port cruise.

And for gamblers, Norwegian Caribbean Lines has a "Sea Saver" plan through June 13, where you pick the weekend you want to depart and the line picks the ship. From April 25, the fare is $589 inside and $659 outside.