For PostgreSQL databases on Digital Ocean, you not only need to select "Use SSL" but also set the profile to "Digital Ocean"and specific a CA Key File - that you can download from the digital ocean admin console for the managed database and save in your /certs/ folder of your project.
Login to Digital Ocean, go to your Database Cluster and use the "Download CA Certificate" option in the connection settings.
See How to Connect to PostgreSQL Database Clusters :: DigitalOcean Documentation
Then save the downloaded file in your project folder under /certs folder (create if needed) and you can select it then from the Use SSL options for the target.
@Teodor will adjust that in our docs Using Managed Databases with the Resource Manager