There is really only one marina downtown on the Willamette, Riverplace Marina. Used to be nice. But that was before all the riots, Covid and homeless. Have not been down there in a while.
Most marinas are on the Columbia or Multnomah Channel. Getting to downtown without a car would be difficult.
I too would recommend Salpare Bay Marina. This was our home base when not cruising up north, for almost 5 years. New owners have already made improvements and from everything that we have heard easier to deal with and more accommodating. We miss Salpare Bay.
Its the last exit on I5 heading north and first exit heading south. Hope they reopened Safeway, But many restaurants, a west marine and lots of big stores on the island. A good place to call home, even if for a short period.