You Do Need power plug adapter while traveling from Saint Lucia 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 Saint Lucia is G. This is how they look like.

Saint Lucia

In Saint Lucia, outlets of type G are in use.

Type G

Marshall Islands

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

Type A

Type B