The light rail runs every 10 minutes or so from the International District/Chinatown stop (a block from King Street station) to the airport, and takes about 35 minutes. Given that, a four-hour delay is the maximum delay that you could make and still comfortably make your flight. There's 18 days over the past 365 days (or about 5% of arrivals) where there was a greater than 4 hour delay. Of those, six were arrivals between 2:25 PM and 3:00 PM, which without checked bags would still be fairly comfortable. There's another 3 arrivals in the 3:00 PM - 3:30 PM range (tight but doable with Pre/Clear, Uber/Lyft/taxi, and no checked bags) two more in the 3:30 PM - 4:00 PM range (very tight but might still make it,) and a 4:21 PM arrival (probably not doable, but still technically before departure.) The other six arrivals are after the flight would have departed.
A lot of this would have to do with your risk aversion and what kind of alternative flight times would be available should you miss your 5:30 PM flight. Even with trip insurance that would cover that short of a connection, if there's no other options once you arrive in Seattle to get to Anchorage before your cruise leaves you may have to try to catch the cruise at another port and miss part or all of your cruise. That said, it looks like there's some late-night options on Alaska on certain days (including an 11:30 PM and a 12:40 AM flight some days) that on all but one day over the past year (and only a couple dozen over the past 10 years that I can find data for) would make the connection.
If the train is severely delayed, you'd feel a lot calmer and less concerned about the delay if you're not worried about making even a late-night flight. I also wouldn't want to have the connection be so tight as to only have one overnight before the cruise after the Seattle arrival; I'd either do two nights in Anchorage or a night in Seattle and a night in Anchorage. If the train is severely delayed and you're having to rebook to the next morning at the last minute, it may be difficult to find availability on the early-morning flights that'd likely be required to make the cruise departure time. Even insurance can only help so much; while they can pay for covered misconnects and delays, if there's no availability in the timeframe you need they can't make that suddenly appear.