(Chapter 9) Footnote 50:

That is, an interpreter who will not drown in Galen’s obscurity. Vesalius probably learned this expression from the Suda, a 10th century lexicon widely used in the Renaissance as a source of Greek medical terminology. The Greek term is Dh/lioj kolumbhth/j, “Delian diver,” recalling an anecdote recorded in Diogenes Laertius Lives of the Philosophers 2.22 in which Euripides gives Socrates a philosophy book by Heraclitus “The Obscure” and asks him his opinion of it. Socrates replies, “The parts I understand are excellent; so, I think, are the parts I don’t understand, but it needs a Delian diver not to drown in it.” Suda Lexicon D 400. The phrase probably referred originally to sponge divers near the Aegean island of Delos.