Concerning immigration, French Guiana has different laws. French Guiana is not part of the Schengen territory. For some neighboring countries Brazil, Suriname, Dominican Republic etc. it is easier to go to Paris than it is to reach Cayenne.
From Brazil and Suriname, to cross the rivers Oyapoque and Le Maroni, which takes 15 minutes, it's not very expensive, but you have to haggle.
From Brazil and Suriname, when you reach the border, you cross the river with a motorboat.
From Brazil you can take a private barge across the Oiapoque River. Its owner can be contacted in St Georges or in Oiapoque. From Suriname, there is a ferry to cross the Maroni River.