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

Here is Why You Need An Adapter

The outlet and plug type used in Puerto Rico 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

Puerto Rico

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

Type A

Type B