The Trondheim fjorden has really clear water and its very nice to dive in unless its been raining really heavily for a few days. There is a lot of big rivers running out in the fjord. All the rivers brings a lot of sediments with them that might cause the water to become a bit more cloudy.
A little 10-minute boat trip from Hindrum Fjordsenter offers fine deep-water corals at 20-25 meters deep. There are also opportunities for wreck diving close to the fjordsenter.