clear-eyed
睟 centers on clarity of vision: having clear, bright eyes that see directly and without cloudiness.
睟 combines 目 (eye) with 卒, which likely contributes the sound. The character is rare, and its exact historical development is uncertain.
The eye 目 on the left is paired with 卒, which looks like a soldier standing straight. Imagine a soldier with clear, sharp eyes scanning the horizon.
For スイ, picture a clear-eyed swimmer (スイマー) whose vision is perfectly sharp underwater: swimmer -> スイ, and the eye 目 stays clear.