If I had only one day in P.V. This is what I'd do. Take a yellow cab or bus to the Cuale early in the morning. I'd spend 2-3 hours shopping in the Zona Romantica, strolling up and down the Malecon, the Cuale and other streets nearby. Don't forget to visit the flea markets in the area too.It is a wonderful section of town for morning shopping and gawking. You can buy a coconut from a vender if you choose. I'd go to Rizo's Grocery store and snoop around too. Look for the GR sign. Mexican grocery stores are very interesting. Don't forget to look upstairs too, it's a hoot. If you are lucky someone downstairs will have a bull's head in the grocery cart, ready for check-out. (I'm not kidding.) Across the street is a little store called "Pie in the Sky." They have killer brownies if your kids have a sweet tooth. Also as you leave Groceria Rizo, turn left and take the kids for an exciting walk over the suspension bridge. It's a good thrill for free.
Then I'd walk just a few blocks to either Le Bistro, El Torito, or River Cafe for lunch. My first lunch pick would be Mariscos Polo but Polo closes on Tuesdays I believe. Le Bistro and River Cafe are both on the Cuale and offer beautiful lincheon views of the river and both are very attractive outdoor restaurants with really good food, safe water, etc.. El Torito is a bit funkier but has amazing ribs and freshly steamed prawns for very little money. It's a great place for a larger group.
Then we'd go just west a block or two to Los Muertos Beach and hang out not far from Burro's Bar under a nice Palapa. There are lots of water sports there or if you go to the "El Molina de Agua Hotel" right there on Los Muertos Beach, they will let you use their pools and facilities for almost no money. (Sometimes for free) There are snorkeling places in P.V. but I don't think P.V. is excellent for snorkeling. If the kids insist on snorkeling, Los Arcos is probably the best place. You can take a water taxi there or get a private tour to there, with equipment, for a half day of snorkeling. Check-out Chico's Dive shop for possible tours.
Then about 4:30 or so I'd head back to the ship via bus or yellow taxi. This will give you a little time for last minute shopping at the little stores by the pier. Then you don't have to worry about being left by the ship and you'll have all the souvenirs you require.
I hope this helps.