2019 Update
As of watchOS 6.0 it is possible to create apps that run on the watch without the need to install a corresponding app on the paired iOS device.
You can follow Apple's instructions to convert an existing watch app into an independent watch app, or just create a new stand-alone watch app in Xcode.
The App Store Connect process is still essentially the same - Create an iOS app. You do not need to provide any iOS app screenshots, only watch screen shots.
Original answer
Apple Watch apps are hosted inside an iOS app, as an iOS extension - so to submit an Apple Watch app you need to submit an iOS app.
From the Apple Watch Programming Guide
Apple Watch requires the presence of an iPhone to run third-party
apps. To create a third-party app, you need two separate bundles: a
WatchKit app (that runs on Apple Watch) and a WatchKit extension (that
runs on the user’s iPhone). The WatchKit app contains only the
storyboards and resource files associated with your app’s user
interface. The WatchKit extension contains the code for managing the
WatchKit app’s user interface and for responding to user interactions.
Because a WatchKit app extends the behavior of your existing iOS app, the WatchKit app and WatchKit extension are bundled together and packaged inside your iOS app bundle. During installation of your iOS app, the system prompts the user to install the WatchKit app when a paired Apple Watch is present.
When you submit your hosting iOS app to App Store you have an option to provide Apple Watch application screen shots to go along with your main app screenshots.
