Answer:
Ned's plan once the Nautilus had entered the Mediterranean was to take the life boat to escape.
Step-by-step explanation:
Ned planned that he was to be at the central companionway with Conseli and told Aronnax to wait in the library until he receives a signal from Ned. He took a monkey wrench with which he planned to unscrew the nuts that secured the life boat to the Nautilus, that action was meant to propel them to reach the surface of the water fast without anyone noticing.
The plan failed though, as the Nautilus travelled very fast once it entered the Mediterranean.