You Do Need power plug adapter while traveling from Cayman Islands to Saint Lucia.

Here is Why You Need An Adapter

The outlet and plug type used in Saint Lucia is G and in Cayman Islands are A and B. This is how they look like.

Cayman Islands

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

Type A

Type B

Saint Lucia

In Saint Lucia, outlets of type G are in use.

Type G