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

Here is Why You Need An Adapter

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

Saint Martin

In Saint Martin, 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