You Do Need power plug adapter while traveling from Namibia to Cook Islands.

Here is Why You Need An Adapter

The outlet and plug type used in Cook Islands is I and in Namibia are D and M. This is how they look like.

Namibia

In Namibia, outlets of type D and M are in use.

Type D

Type M

Cook Islands

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

Type I