You Do Need power plug adapter while traveling from Guinea-Bissau 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 Guinea-Bissau is C. This is how they look like.

Guinea-Bissau

In Guinea-Bissau, outlets of type C are in use.

Type C

Marshall Islands

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

Type A

Type B