I'll echo...check your health insurance policy. Some cover expenses out of the US, some don't. Some will cover a helicopter evacuation, some won't.
A helicopter evacuation from the ship can run $20-25K depending on exactly where the ship is, etc. $500,000 is a lot of insurance--consider that many policies have a 1 million lifetime benefit. However, 50,000 is an amount that can very realistically be reached in a few days, especially if it involves an evacuation.
Reality is that you need coverage for just about anything that can happen for 24 hours. By then, you will probably be back in the US. However, again check--will your health insurance kick in as soon as you are "back home?" I know it gets picky, but things can happen. That's when the insurers bring out the fine print!