Internal/Sound/Ambient/DoodadEffects/WaterLakeLoop1.wav
Internal/Sound/Ambient/DoodadEffects/WaterStreamLoop1.wav
Internal/Sound/Ambient/DoodadEffects/WaterWaterFallLoop1.wav
Internal/Sound/Ambient/DoodadEffects/WaterWavesLoop1.wav
EDIT: made a tutorial (lol):
firstly open sound editor. find one of the soundloops that you like, and select "use as sound". this will be your sound variable.
next go to toolpalette. select region palette. create a new region, where you want the sound to be heard and select "edit region properties".
now a new window opens named Region Properties. tick the "Ambient Sound" and select the suitable sound variable. click OK and CONGRATULATIONS! you have just created a sound loop.
click test game (CTRL+F9 i think) and listen to it. i think you can turn the loudness up at the sound editor by right clicking the sound variable and editing its properties.