I am sufficiently uncomfortable with dealing with a non-PEC-waivered policy that I will always buy one with a PEC waiver. That said, if I understand lookbacks correctly (and I would call the insurance company to verify this):
The lookback refers to the number of days back from your date of purchase they "look back" to see if you or your family members have been treated for any illness, or consulted for any illness, or had a change in your medication.
So, lets say that you, your spouse and your children are perfectly healthy, and you're packing for your cruise and getting ready to go to the airport when you get a call that your mother, who has been ill for years, has died, and you will not be able to take your cruise.
Do you get your money back? No.
Because your mother was ill during the lookback period, her death is not a covered event.
On the other hand, let's say you and your family have been perfectly healthy, you buy the insurance, you're packed and ready to go to the airport, and your oldest son doubles over in pain with appendicitis.
Do you get your money back? Yes. Because your son was not ill during the lookback period.
Me, myself, I think it's just too dangerous to travel without a PEC waiver. They could decide anything for which you were treated, from a hangnail to a wart, contributed sufficiently to your present condition that they could deny the claim. You could fight it, but your court costs would probably exceed your financial return It's just better to be safe, IMHO.