Based on a quick look at the documentation, i think the API Waterline exposes to clients is not an imitation of the MongoDB API, which would allow PostgreSQL to be used as a drop-in replacement for MongoDB. Rather, it's its own thing.

Waterline does support both MongoDB and PostgreSQL, though, so you could use it as part of a migration strategy: start with MongoDB, install Waterline backed by MongoDB, migrate your app to use Waterline rather than using MongoDB directly, replace MongoDB with PostgreSQL.