In order to register Owlet and remove watermarks you need to purchase it first. Then, use Help -> Registration menu or the link in the trial reminder window to complete the registration process. You will need a serial number for that. Normally, you should find one in your purchase confirmation e-mail, if you don't - please contact our support team.
Copy your serial number from the order confirmation e-mail to the clipboard and paste it to the Registration window. Click OK and restart the application.