if I should be worried about interference from the walls
Depends on the actual geometry of your setup.
Yes. Easy to test.
The ultrasonic is going to "hear" the first reflection back to it.A ribbed culvert will bounce the signal right back, while a smooth wall tube will might carry the sound far, far away.
Why placing it in such a tube in the first place?
Depends on the material used and the wavelength of the light. Light sounds like a better solution as you for sure won't have problems with the walls.Usually IR is used, that will indeed shine through water but be reflected by opaque liquids like milk. Powers will generally be no problem, they reflect in all directions so also back to the sensor.
You have to narrow it down.
try ir bouncing off a float.