You Do Need power plug adapter while traveling from Cayman Islands to Sri Lanka.

Here is Why You Need An Adapter

The outlet and plug type used in Sri Lanka 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

Sri Lanka

In Sri Lanka, outlets of type G are in use.

Type G