Yes the missing slash was a cut and paste error. At the bottom is the full correct command.
I am using the same password and same username for mongosh. (Just to belay a possible distracting question, the tests were repeatedly scripted using the actual strings, so we could verify that they were the same values used.)
It would follow from the suggestion, that mongodump uses a different authentication mechanism.
Is this likely to be the case and if it is, how is this configured.
mongodump --username smontsaroff --password $MONGO_SERVER_PWD mongodb://nwcdbd26.nwfsc2.noaa.gov:27017 --db boatnet_sandbox --archive=boatnet_sandbox.20220824.archiv
2022-08-24T17:58:22.041-0700 Failed: can't create session: could not connect to server: connection() error occurred during connection handshake: auth error: sasl conversation error: unable to authenticate using mechanism "SCRAM-SHA-1": (AuthenticationFailed) Authentication failed.