Light fittings melted, walls were left charred and facilities scorched after a blaze was started inside the men's toilets.
Guernsey Police is now on the hunt for those responsible and Crimestoppers is offering up to £1,000 for information that leads to an arrest and conviction.
The damage is believed to have been caused early on Thursday.
'This is the worse case I have seen in many years,' crime reduction officer Andy Goodall said.
Anyone with information is asked to contact police on 725111 or anonymously through Crimestoppers on 0800 555 111.
