We ended up doing BUF - SEA in 2017, through JFK going out (JetBlue) and BWI (SW).Weird to go East an hour to go West, but we had major points on JB that basically got two of us to SEA for free. I am a ruthless planner, discovered that at one point we could have flown YYZ to SEA on Delta for about the same price. Part of it will come down to hte day of the week your cruise will leave. For 2017 we did Carnival Legend that left on a Tuesday (yes the same Carnival Legend involved in the accident today in Cozumel) If you are flying down for a weekend departure, it will be harder to get a deal.
One of the issues is that coming back we didn't get in to BUF until midnight. With kids in tow (they were 13 and 9 at the time), we had a hotel booked and crashed out for the night, driving back to Toronto in the morning. If we had flown in to YYZ we would have been in our own bed (and the additional hotel cost could have gone to airfare from YYZ).
If you haven't already download Hopper and get its recommendations and get it tracking flights for you. Used it the last couple of years, most recently for our San Juan and New Orleans flights for the upcoming cruise in February. Also have it set up for the RT Alaska cruise from Vancouver we have at the end of August.