sdsearch

If the OP doesn't have experience with cruises, that's not enough information to avoid a dangerous problem. And there's another thing that's incorrect about the cruise destination (explained below).

It should be:

• Fly them to Vancouver to arrive at least a day before the cruise leaves.
• Book them a hotel in Vancouver that can be cancelled without loss of too much money if their flight is late (it's to allow for that that you're arriving a day early!)
• Book them passage to Seward (usually cruises don't go to Anchorage proper, the water's too shallow there for big cruise ships, they have to go to a commercial port far from the city which most don't want to do)
• Book them a connection (Alaska train is nice) from Seward to Anchorage (but maybe allow time in Seward for a local day cruise on Major Marine Tours). Provide plenty of time for this land connection too (see below).
• Fly home from Anchorage.

As you see, it's much more complex than you proposed. There are no plane flights from Seward, and no cruises to Anchorage, and it's at least a 2-hour drive (but on a two-lane road which can close for hours at a time in case there's fire right next it, like when I visited last month). And if you don't buy the plane ticket to Vancouver from the cruise company, you're not "protected" in case you miss the cruise because of a plane delay.

I was once on a plane DFW-LAX. One engine failed, and the plane diverted to Lubbock, where at the time AA didn't have facilities for full-size planes. It took an hour or two to get the passengers and then luggage off of the disabled plane, and it took over 7 hours before the replacement plane arrived and we able to continue. In my case, I was just flying home nonstop, so no big concern as to exact timing. A bunch of people in the waiting area had been flying to meet a cruise, tho, and of course a bunch of them didn't follow the above rule and now with this delay of "only" 7 hours were going to miss the cruise. Those who had booked the flight through Disney (the cruise operator in this case) were on the phone getting rerouted to join the cruise in progress (Disney would fly them to a new location). But those who had booked the flight separately had no clue what to do (and I bet many of this simply missed the cruise), because all they could do was complain that neither the air carrier nor the cruise line would do anything for them (without paying lots of extra money). In the event of delay like this, the plane carrier only has the responsibility to get you to where the ticket says you were going, so if all it said was LAX, that's as far their responsibility goes.

Oh, and of course, they need up-to-date passports to connect air-to-cruise in Vancouver, BC, Canada. Since the OP was asking only about flying to Alaska (which is in the USA), we don't know if they have current passports.