In 20,000 Leagues Under the Sea, why are the men so pleased with the coconuts?

In chapter 20 of Jules Verne’s adventure 20,000 Leagues Under the Sea, Captain Nemo permits his captives to visit the coast of Papua New Guinea after a lengthy period at sea. Professor Aronnax, Ned Land, and Conseil eagerly row their small boat to the shore and revel in the feeling of solid land beneath their feet. Ned Land is particularly interested in collecting food, as he is tired of the diet of seafood served on the Nautilus. Here is the moment you describe in your question:

. . . the Canadian . . . discovered a coconut tree, beat down some of the fruit, broke them, and we drank the milk and ate the nut, with a satisfaction that protested against the ordinary food on the Nautilus.

The trio then collects a boatload of coconuts and brings them to the Nautilus, where they store them for future consumption.
This short episode is interesting because it reminds the reader that Arronax, Land and Conseil are captives. With the wondrous undersea adventures the characters have on the Nautilus, it can be easy to forget that they are jailed on the submarine by Captain Nemo. Eating the coconuts reminds these characters of the joys offered on solid ground. After the visit to the coast, Ned Land in particular begins to chafe against his captivity and wishes for freedom.