To get the waiver of pre-existing conditions, the clock starts ticking
as soon as you make the first payment of any kind for the trip (refundable or nonrefundable) - which is usually the cruise deposit. Look carefully at the policies, because
some also take into account the pre-existing conditions of your family members. I get mine from the Trip Insurance Store (
www.tripinsurancestore.com), and I usually go with Travel Insured or Travel Safe.
If more than 2 people are traveling, make sure that you fully cover each person, rather than the total trip cost. Since the first 2 people in the cabin pay more than 3rd and 4th passengers - you may not fully cover the first 2 people if the amount of coverage equals the total trip cost.
EXAMPLE: Passengers 1 and 2 pay $2,000 pp and passengers 3 and 4 pay $1,000 pp. The total trip cost is $6,000. If you buy $6,000 in total coverage, each person gets $1,500 in coverage - which under insures the first 2 people by $500 pp and over insures the 3rd and 4th passengers by $500 pp. It's best if you can choose different amounts of insurance coverage for each traveler.
Insurance is usually priced in $500 increments, so I round up to the next $500 increment. If my cruise costs $1,750 pp, I round up to $2,000 pp in coverage - usually for the same price.