Basically what the title says; if there's carpet on top of ice and you ride a boat in it, the ice physics are not applied to the boat and you won't go fast. However, ice physics are still applied to you and dropped items, and you and the items still slide on the ice.

Last time I played on Legacy Console (Wii U Edition), boats could still go fast on carpeted-covered ice, so I'm assuming this is a bug here.

Recreating this is easy, just make a long stretch of packed/blue ice, put carpet on top, and try to use a boat on it.