Personally, I wouldn't bother with a beach excursion. St. Maarten offers one of the most convenient beaches near a cruise port in the entire Caribbean.
Great Bay Beach lies in front of the Phillipsburg Boardwalk and the city's tourism district. I recommend this option if you want to keep your visit cheap and straightforward. Save your excursion money for something else.
You have three options to get there from the cruise terminal:
1. You can walk it for free in about 15 minutes. This is the best option for tight budgets, but do it only in comfortable temperatures.
2. You can take a cheap water taxi. They usually make several stops, but they offer great views of the bay and St. Maarten.
3. You can take a slightly more expensive land taxi. It's the quickest option.