I was just made aware that the original film that was released for the book actually mentioned a Great Lake. Apparently, at the time, Lake Erie was in terrible condition and highly polluted, which prompted the Lorax line
Let's say you did want to visit them all. First off, you'd have to be retired because it would be a 7-day-a-week adventure. If you visited 5 islands a day every day of the year, you would finish visiting all of them in about 20 years.