Project Gutenberg has the 1991 Frederick Paul Walter translation which he dedicated to the public domain. Note that this is not the version that has been so widely praised, but does form the backbone of the completely restored and annotated edition 1993 translation by Walter James Miller and Frederick Paul Walter. It's also known as
Twenty Thousand Leagues under the Seas, a title F.P. Walter felt was closer to the original. The 1993 version is published by the Naval Institute Press and is not available in ebook form.