In a previous life I was the service manager at a Mercedes/Porsche/Rolls Royce dealership. The sales department LOVED selling those things as the markup is huge (inside tip: if you do want one haggle like crazy, you can probably get it at 50% off). They also sold a version on dealer certified used cars.
For those that had them and really, really needed them for something big it was probably a good deal. One blown up transmission could mean a net savings. But 99% of car repairs are less than catastrophic and I'm sure (no actual data, just my gut feeling) the huge majority of the purchasers never got anywhere close to a good value. Which is why the insurers manage to stay in business.
As others have pointed out, the actual cost would be huge on a used boat. My view is that the money can be better spent on doing regular preventative maintenance religiously, getting professional help when needed, and knowing your boat inside and out so that you can detect right away when a problem is developing and can take care of it before it turns into a large one.