Manx Utilities says a contamination incident involving water supplies in Lower Douglas last week involved the Ben my Chree.
Sea water was accidentally pumped into the fresh water supply as workers washed down the Steam Packet vessel during a routine procedure.
Steam Packet Chief Executive Mark Woodward says connecting to the fresh water supply has never caused problems before - but a backup pump kicked in, forcing harbour water into the system.
Manx Utilities technicians have fitted a non-return valve to the system to make sure such an incident can't happen again.
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