You Do Need power plug adapter while traveling from Montserrat to Croatia.

Here is Why You Need An Adapter

The outlet and plug type used in Croatia are C and F and in Montserrat are A and B. This is how they look like.

Montserrat

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

Type A

Type B

Croatia

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

Type C

Type F