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

Here is Why You Need An Adapter

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

Dominica

In Dominica, outlets of type D and G are in use.

Type D

Type G

Saint Martin

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

Type C

Type E