Thanks to Steven Rice for the photo and report:
This is my first sighting of a “Repair, Berthing, and Messing” barge. It passed by Alki Point around 11:30 am presumably heading to Bremerton, but I didn’t get to see if it turned that way.
Here’s a link with basic info. And my shot from up near the west side of Schmitz Preserve.
An archives check brings up another reader photo of a YRBM in the area back in 2017.