You Do Need power plug adapter while traveling from Tonga 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 Tonga is I. This is how they look like.

Tonga

In Tonga, outlets of type I are in use.

Type I

Saint Martin

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

Type C

Type E