Check your cruise line for price/flights. Flights are released 330 days from disembarkment date.
I am flying non stop YYZ-FCO in Nov. I have Princess EzAir for the flights. The day EzAir flights came out I booked Flexible flights - can modify/cancel with no $ penalty up to 45 days of cruise and no payment until final payment of cruise. Princess EzAir also offers- Restricted - pay when booked and $ to cancel/modify. I have Premium Economy seats on Air Canada. I check my price daily. One day, I noticed the price had gone down from $3,200 to $2,079/pp for Premium Economy. Today, the same flights are $4,150/pp for Premium Economy, $1,867/pp for Economy.
I would book with your cruise line, watch the price and when it gets close to paying check the price with the airline and take whatever is cheaper.