Where to stay in Amsterdam before river cruise?

River cruises dock in several different places in Amsterdam. The docks are towards the northern part of the historical city center, whereas most tourist sights are more towards the south.
It’s therefore a bit difficult to suggest a hotel that meets all your criteria.

Thank you! I've contacted the river cruise company to see if I can get more information about the departure point.

Would it be difficult to get to the cruise departure area if we stayed in the historic city? Is it possible to take public transportation to the harbor?

I agree with the sentiment that says to stay someplace interesting and close to the sights you want to see, then take a taxi or public transport to the docks. Unless you know specifically where the ship will dock (they can be spread out, and Amsterdam is always making adjustments to where cruises can dock) it may be difficult to choose the right spot, and the hotel selection limited.

If you want classic Amsterdam, then something along the central canal belts, like the Jordaan area. Many also like the Museumplein area. If these areas appeal, make your reservations early, especially if a weekend, and be prepared to pay a hefty price.

Conversely, if Amsterdam proper is not your main interest, staying in Leiden, Utrecht, or Haarlem is perfectly fine, even further afield like Delft or Den Hague works. I also have stayed a number of times at hotels near the Sloterdijk train station, a few minutes from Centraal station, a nice compromise between price and easy access to central Amsterdam.

Would it be difficult to get to the cruise departure area if we stayed in the historic city? Is it possible to take public transportation to the harbor?

First off, I'd agree with @paul's comments. Also, there is relatively easy to navigate public transport in Amsterdam with Central train station being a primary start/end point and the river cruise berths.

But secondly, Amsterdam's "historic" city - what I consider most of the museums and sights to see in a couple of days - is pretty small and easily navigable such that finding a well-located hotel is mostly about price range. It is going to be a snap for you to get to the river cruise departure area from pretty much all of Amsterdam (and relatively many nearby towns/cities), so you can just look for the hotel that meets your budget and that says "looks nice" to you.

I've liked the two places we've stayed in Amsterdam - the very "easy" Marriott and the Kimpton de Witt. Those remain our "go to" hotels for return trips (points/status members), but there are about a dozen other hotels I have saved off for Amsterdam due to recent recommendations from folks here.

Dig around in the threads here, and you'll find many recommendations for Amsterdam that should work fine for you.

Whilst on the RS Belgium/Holland tour in April, we were booked in Dikker & Thijs Hotel during our Amsterdam portion and liked it.

We stayed here and liked it a lot: https://sweetshotel.amsterdam/locations/sweets-hotel-amsterdam-kortjewantsbrug/

I've contacted the river cruise company to see if I can get more information about the departure point.
Would it be difficult to get to the cruise departure area if we stayed in the historic city? Is it possible to take public transportation to the harbor?

Well, if it's Viking Cruises, Their dock is behind the Doubletree hotel, near the train station. We stayed there on one trip and walked our bags to the port the next morning. We took public transportation everywhere, but until you know where the dock is, it's hard to say what's possible or not.