I agree with boatsie. If an independent travel insurance company won't insure an operator, it would be unwise to book with that operator. Of course, if you can secure another insurer who will cover default from that same operator, you shouldn't have much to worry about.
I am sailing on the NCL Sun this month and purchased my travel insurance through Travel Guard. The protection includes operator default. Travel Guard insured me with no problem or questions. So, I guess that's a good sign for NCL. I also paid by credit card, which provides extra protection.
If you purchase independent insurance and use a credit card, there are other issues (security, terrorist activity) that may be riskier than losing money due to default.
A final thought, even with insurance and credit card protection, if any line ceases operations there are inconveniences to be considered. Having to rebook or (worse) enroute disruption can be aggravating.