You Do Need power plug adapter while traveling from French Guiana to Marshall Islands.

Here is Why You Need An Adapter

The outlet and plug type used in Marshall Islands are A and B and in French Guiana are C, D and E. This is how they look like.

French Guiana

In French Guiana, outlets of type C, D and E are in use.

Type C

Type D

Type E

Marshall Islands

In Marshall Islands, outlets of type A and B are in use.

Type A

Type B