You Do Need power plug adapter while traveling from Monaco 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 Monaco are C, E and F. This is how they look like.

Monaco

In Monaco, outlets of type C, E and F are in use.

Type C

Type E

Type F

Saint Martin

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

Type C

Type E