If your initial payment is also your final payment, you should be able to purchase insurance which covers pre-existing conditions. Just about all of them should work--ironically, in this one corner case, CSA has the shortest window--within 24 hours of your final payment.
Whichever insurance you choose, though, I'd recommend buying it the same day you make your payment. That way, if anything develops between that date and your cruise, you're covered.