I buy through tripinsurance.com because it covers everything including flights to and from port that I book and any personally booked shore excursions. It is also cheaper (about half less) than HAL's plan even at the top tier cancel for any reason coverage.
I do think that you need some form of travel insurance though. Two years ago my dad tore the ligements in his knee halfway through a Transatlantic. The HAL insurance paid for Business Class tickets for him and my mom from Portugal back to DFW because his knee had to stay extended. This was over $12,000.