Yes, you can talk to Siri on your watch. And with an Apple Watch Series 3 or later, Siri will even talk back to you. Long-press on the Digital Crown, and Siri will appear, awaiting your command or question. With watchOS 7 or higher, you can make a greater variety of requests to Siri, such as translating a word or phrase.
You can also get Siri's attention by voice. On your watch, go to Settings > General > Siri. Enabling Hey Siri lets you say "Hey Siri" to your watch. Enabling Raise to Speak lets you raise your watch to your mouth where you can then say "Hey Siri" to chat with the voice assistant.