The Harbour Master’s Mother is mentioned a few times in the series.
In the Season 2 episode Emily and the Rocket, he accidently calls her bossy on the phone when he meant to call The Oil Refinery Boss. The Harbour Master tried to explained to his mother what happened.
She was also mentioned in the Season 5 episode, Bedford's Big Move, where she sends him a box of tugboat cookies in the mail. At the end of the epiosde, the Harbour Master telephones his mother to thank her for her kindness.