If you have conditions for which you have been treated that could flare up and either prevent you from cruising or require medical care during the cruise, then you need insurance that covers pre-existing conditions. Many policies will automatically cover pre-existing conditions only if the policy is purchased within a few days of your initial payment.
If you are not concerned about covering pre-existing conditions, then you can wait until you have non-refundable expenses. This is typically at your final payment date, when penalties for cancellation begin. If you want to cover airfare or non-refundable hotel or car reservations, you may want to get insurance as soon as you purchase those.
The main risk in waiting to purchase insurance is that you could develop a condition that would count as pre-existing between now and the time you buy insurance. In that case, if you need treatment for that condition during your cruise, for example, it might not be covered unless you buy (more expensive) insurance that does cover pre-existing conditions.
