You Do Need power plug adapter while traveling from Puerto Rico to Saint Lucia.

Here is Why You Need An Adapter

The outlet and plug type used in Saint Lucia is G and in Puerto Rico are A and B. This is how they look like.

Puerto Rico

In Puerto Rico, outlets of type A and B are in use.

Type A

Type B

Saint Lucia

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

Type G