If the Dutch side of St. Maarten mirrors the Netherlands then I assure you that third-party liability insurance is included in your rental agreement. I've never asked this question in the past when renting a car on the Dutch side, and I personally turndown every 'extra' offer they try to throw at me. And, yes ... they sell most of these items just to make money. The same as when you buy a new car and they attempt to sell you every 'pack' item you don't need from lifetime wax finish to paintless dent repair. The money that is made on these useless extras is stunning. About 20 percent or so of people who visit Europe (not sure about the Caribbean) purchase extra liability insurance coverage that they simply don't need. That's a lot of wasted money, IMO.

Let's look at where you live. If you have automobile insurance with Allstate of Canada (a company that I am very familiar with), and you tell them that you are traveling to Europe for two weeks and plan to rent a car while your there. I assure you they will try to sell you a personal umbrella policy to cover all your driving needs when in Europe, when in fact the vast majority of this coverage would not be needed. It will be expensive, and these make a handsome profit for the insurance company. Remember that the primary principle in auto insurance is that the coverage follows the car, and rental car agencies must carry insurance limits to cover the liability for those who rent and use their cars. If there is something unconventional that car rental companies do on the Dutch side of St. Maarten I'm simply not aware of it. But, when renting a car there if someone is unsure let them know that it is your belief that liability insurance should already be included in the rental agreement.