There are better translations out there nowadays. I think I've shown you my copy of the first volume of The Annotated Jules Verne -- which is, in fact, 20,000 Leagues -- and the translator/annotator starts with the standard, 19th-century translation, but adds corrections, makes footnotes about the original mistranslated passages, and restores entire chapters that were excised.

He also has an extensive forward and afterward, castigating the original English translator in great and scathing detail.

Since then, more and more new translations of Verne have come out -- hopefully, more scientifically literate ones.