You Do Need power plug adapter while traveling from Pitcairn Islands 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 Pitcairn Islands is I. This is how they look like.

Pitcairn Islands

In Pitcairn Islands, outlets of type I are in use.

Type I

Marshall Islands

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

Type A

Type B