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

Here is Why You Need An Adapter

The outlet and plug type used in El Salvador are A and B 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

El Salvador

In El Salvador, outlets of type A and B are in use.

Type A

Type B