But what do we do with the data showing that many kinds of zooplankton don't dive deep enough during the day to become invisible to predators or that others dive deeper than it's necessary to escape hunters' eyes.

根据我对《剑桥语法》中比较分句结构的理解，这个句子是个错句。than分句应该是：

than is necessary to escape hunters' eyes.

因为than分句必须留出一个gap（空位），作为比较短语的对应项。即比较分句必须是结构不完整的分句。deeper的对应项必须在从句省略，而省略的对应项是不定式to dive的状语。不定式省略后，形式主语it必须同时省略。to escape hunters' eyes只不过是to dive的目的状语，并不是than分句的真正主语。所以，原句的比较分句结构错误。补全的分句为：

than (it) is necessary (to dive       ) to escape hunters' eyes.

真正主语省略了，形式主语it必须同时省略。