> On the other hand I remember a particularly tedious passage in 20,000 Leagues Under the Sea where several pages passed describing various molluscs found at the bottom of the sea.

This reminds me of the passage in The Picture of Dorian Gray which amounted to some detailed description of gemstones or worse, the chapter in The Great Gatsby that was essentially a list of names.

I usually aim to read a text in its entirety out of respect for the author, but I had no qualms fast-forwarding through those sections.