Highwater Trousers' activation code is a holotape used to activate the Highwater-Trousers defense program in Fallout 3.
Location
It can be found on the female ghoul wastelander in SatCom Array NW-05a.
Transcript
X948KJ
L4IERU
ER32J7
Notes
The wastelander also carries the two SatCom array coordinates holodisks.