This can be solved by adding the following annotation to the UUID:
import org.hibernate.annotations.Type;
...
@Type(type="pg-uuid")
private java.util.UUID itemUuid;
As to why Hibernate doesn’t just make this the default setting, I couldn’t tell you…
UPDATE:
There still seem to be issues using the createNativeQuery method to open objects that have UUID fields. Fortunately, the createQuery method so far has worked fine for me.