Covert.stealth.underwater is the skill used when sneaking underwater.
When sneaking, this skill is checked against anyone in the room's adventuring.perception to determine whether they saw you move or not. It does not determine their ability to actually see you in the room, however, as this is decided by covert.hiding.person.
This skill is used by the Ninja when using a chop inhume and is tested against the other victim's adventuring.perception, if for some reason they happen to be inhuming someone underwater.
This skill determines if potential anglers are noticed by the blowfish in the pond in the ninja guild.
No groups have this skill as a primary.