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

Here is Why You Need An Adapter

The outlet and plug type used in Costa Rica 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

Costa Rica

In Costa Rica, outlets of type A and B are in use.

Type A

Type B