1. What does it mean?
잠수 literally means diving underwater. Online it describes someone who stops responding or stays logged in without actively participating. 잠수 타다 can also mean disappearing from messages or social contact for a while.
Also written or said as jamsu
2. The nuance
Absence does not prove bad intent: the person may be busy, disconnected, or taking a break. In a team game, leaving others without notice can still cause problems.
3. Examples
“물 마시러 잠깐 잠수할게.”
I'll be away for a moment to get some water.
“단톡에서 며칠 잠수했더니 메시지가 쌓였어.”
After a few days away from the group chat, messages had piled up.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.