Summary:

• Freight transport to / from fish farms - if the vessel is also used for debugging or towing - no exemption!
• Passenger transport in the form of round trips falls within the exemption
• Ambulance vessels within the exemption

Attached is the complete statement and must be read in order to get a complete picture:

https://www.skatteetaten.no/rettskilder/type/uttalelser/prinsipputtalelser/grunnavgift-pa-mineralolje-mv.--fritak-for-mineralolje-mv.-til-gods--og-passasjertransport-i-innenriks-sjofart--praksisendring/