For sure mines we one of the main threats in coastal waters - as I had mentioned. There were also times, in which the North Sea was not yet heavily mined - and there were still no attacks on any fortified base, e.g. Heligoland.
You wrote that Fisher's ships would be immune to land-based weapons, but typical coastal defence included mines. And for sure, Fisher's ships were not immune against 30.5 cm or 42 cm coastal batteries (and therefore avoided to get in their range!). Fisher's battlecruisers were sunk by smaller 28 cm guns. At the Dardanelles, the ships were bombarded with much older guns, but still suffered heavily. Remember, that Blücher was sunk in 1940 by old 28 cm guns and old torpedoes (all land-based). Ships in the first half of the 20th century were not immune to land-based weapons.