How to visualize Protocols and Extensions in UML?

It seems reasonable to use UML Interfaces to visualize Swift Protocols in UML. But how should I visualize an extension that provides a default implementation for a specific protocol? Should I just use a class like <<extension>>ProtocolName that inherits from that protocol?

An extension exists in the scope of a certain class and just adds some functionality to it. So I would represent the extension as subclass (eventually packaging might be preferred). To show that it's an extension I would use a stereotype. The dependency on the class which is extended is somewhat optional since in the context it is a naming convention.

If the extension will additionally adhere to some protocol you just add realization relations to the according interface classes.

This is one way to express this. Since there is no native UML construct for an extension you are relatively free to invent you own idiom here.