Hi Kim ... typically the cruise ship's shore excursions will be much more expensive than planning this yourself. Also, we've been on a few cruises to St. Martin in the past and I can't recall seeing a shore excursion to PInel. Different cruise lines offer different excursions, but I've never seen one to PInel and back on Princess or Celebrity. But, with that said if your time frame is from 7 a.m. to 4 p.m. you'll need to plan this well, but it shouldn't be a problem. If you're off the ship by 7:30 bear to the right after you leave the cruise port and you'll find a taxi circle. It's really not hard to find. A taxi for two to Cul-de-Sac on the French side of the island should be $25 USD and no higher. Get a quote from the driver before you accept the ride. Let them know that you are planning to catch the ferry to Pinel Island and they will drive you to Rue de Cul de Sac and 'Ferry pour Ilet Pinel'.
It will take about 25 minutes or so to get there, so my guess is you'll arrive around 8:30 a.m. The cost of the small ferry boats to Pinel was $12 per person the last time we did this, but I'm not sure what they are presently. The ferry rides depart every half hour, but before exiting the taxi ask the driver if he or she can return to the same spot that afternoon to take you back to the cruise port. You should try to make this arrangement in advance just in case you have difficulty finding a taxi when you return from Pinel. I would suggest maybe 2:30 p.m. that afternoon. The boat to PInel is fairly quick (10 to 15 minutes), and if all goes as planned you'll be on there by 9:15 or so. You can rent beach chairs and an umbrella at either Yellow Beach or Karibuni. And, you can enjoy a nice lunch there as well. You can even take a hike to the far less crowded north-facing beach, but be sure to wear sneakers if you plan to do this.
Check with Pinel Ferry when the last boat returns to Cul-de-Sac. This should not be an issue for you, and I would suggest catching the ferry back from PInel around 1:30 to 2:00 at the latest. Overall, that should give you four hours or more to enjoy this lovely little island. Overall cost depends if you plan to eat lunch, but between the taxi, ferry boat, and beach chairs rental you're looking about $80 for two of you. I doubt you would find a shore excursion with the cruise line that would be this low. Good luck and I hope this works out well for you.