For completeness - here’s an example: https://www.openstreetmap.org/relation/3995755.

From the imagery, it looks like an area of rock that forms a well-defined river channel. It’s probably usually mostly dry with a small stream making its way through (see the Bing imagery) but occasionally a raging torrent.

The question, I guess is whether should be mapped one object or two. Is it only one (something that is sometimes wet and sometimes not), or two (the rock, and the water that’s there part of the year)?

However it gets mapped, there are ways for data consumers to decide what they want to show here. It’s much easier (using pretty much any rendering mechanism) if both sets of tags are on one object, but also possible if they’re not. Personally I don’t think “how easy it should be to render” should be the main factor here - it comes down to “do you think there is one object here or two”.