Lonely Planet review for JAL Voyages
French Guiana's pristine jungles are impenetrable and dangerous without a good guide. There are several respectable Cayenne-based tour agencies that run a few of their own tours, but more often they hire out guides throughout the country and take a commission on their services. The better of these include JAL Voyages, whose most popular jaunt is an overnight on a floating carbet in Kaw.
A cheaper tour alternative is to go directly to local guides in the specific region you want to explore; you can easily find guides like these at places of lodging throughout French Guiana.