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

Here is Why You Need An Adapter

The outlet and plug type used in Saint Lucia is G and in Saint Barthélemy are C and E. This is how they look like.

Saint Barthélemy

In Saint Barthélemy, outlets of type C and E are in use.

Type C

Type E

Saint Lucia

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

Type G