To do what? Sweep the decks and polish the brass?
I'd guess this is a typical annual boiler inspection and cleaning. Those types of steam powered machines need routine maintenance and occasional rebuilding.
What's more interesting is... Because WDW no longer has a second big boat to use, and they closed the Canoes and Keelboats decades ago, the Rivers of America will be inaccessible that week. That's a huge chunk of the park's property to allow to go unused because they've only got one boat left.