May is less likely to result in delays (less likely to have bad weather, but not impossible--we have had bad rainstorms), but I would leave at least one flight later if you can. Also, you won't want to take the Sounder when you get into the station--you'll want to take the light rail--make sure you're on the correct side of the tunnel, or it will take you in the wrong direction. I haven't taken the train to or from Vancouver, but almost every time I took Amtrak in Northern California, Washington, Oregon or Southern California, I had delays ranging from 30 minutes to 2 hours on short trains (<4 hours), and 1 to 8 hours on long trains.
It's possible the Vancouver train is better about that (if anyone on the list has ridden it regularly, they might know better than me), but I know that, at least on the US side, freight trains take priority over passenger trains--it almost inevitably leads to delays.
Regardless of how you travel, there can be delays in customs, depending on events on the ground in the United States and Europe, and that can delay any travel between the US and Canada--in extreme cases, for several hours. If it's possible, it might be better to take the train the day before, and stay in Seattle that night. Whatever your choice, I hope you all have a wonderful cruise!