I also have wondered about Amtrak being considered a common carrier. When I was planning a cruise and return from Seattle via Amtrak, I called the insurance company and asked that question. The lady hemmed and hawed and without conviction said yes, Amtrak is considered a common carrier. I was uneasy about it, but fortunately, there was no reason to file a claim.
Be careful of any restrictions. I believe you have to arrive the day before the cruise ship departure (no more, no less), or the insurance doesn't cover a missed connection.
Wow, that's really ridiculous. For cruise ship insurance you really have to be in the city/town the cruise is departing from, one day before? That's a silly as crap requirement, for insurance. But I'll admit I'm not as knowledgeable on insurance related things, as others probably are. I wonder if Allianz(sp?) has such a requirement, if you're doing the reverse(cruise ship to connecting to Amtrak in Seattle)?
Anyway, I don't feel there's much of a point to having insurance, for Amtrak trips. But that's just me. And though I have sometimes bought non-refundable tickets if I wanted to travel cheaply on Amtrak, that one would just be best to buy a refundable ticket on Amtrak just in case they can't travel at the last minute, sans Allianz insurance.
I imagine that particular restriction is mainly to help reduce the chance of a misconnect creating a very expensive payout. If a misconnect or significant delay causes a hotel room to be missed for a night, that likely would be, at most, a couple hundred dollars. A cruise is often over a thousand dollars.
That being said, the normal policy I've seen for a misconnect to another form of transportation requires 12 hours between the two, with some generous policies offering a 6 hour minimum. (That's what I was finding when I was looking for coverage for my Canadian trip, anyways, when I had a same-evening departure out of YVR.) Even with a 6-hour minimum, you'd still be looking at arriving at 9 AM if the boat leaves at 3 PM. (I'm not sure how common it is to have a late evening departure, but from what I've seen/heard is that it's typically an afternoon departure.) A 12-hour minimum would basically require arriving on a red-eye flight to be eligible. I'd guess that, in that case, they'd just write in that you have to be in "the day before" a cruise to ensure there's less confusion and to allow a full overnight in case something goes haywire.