Hi there bojo_cl,
To begin, we'd advise against using compressed air to clear water from your Apple Watch:
Keep your Apple Watch clean and dry. Soaps, cleaning products, abrasive materials,* compressed air, ultrasonic cleaning, and external heat sources can cause damage and shouldn't be used with your Apple Watch.
Learn more here: How to clean your Apple Watch - Apple Support
You'll also want to follow the additional guidance in the link above for the best way to clean and dry your Apple Watch.
If you continue to have issues with water remaining in the speaker afterwards, you'll want to reach out to Apple Support directly for further assistance. They're best-equipped to take a closer look at the issue and provide some additional guidance. This link will help you get started: Apple Support
Take care.