Q:

Can I use a ultrasound sensor to measure water level?

If I use an ultrasonic sensor will it detect the water level? 
I was thinking about a product to read water level on water boxes (common in Brazil). I researched about instrumentation for this measure, and I think that an ultrasonic sensor is the best option. Will the water correctly reflect  ultrasound and not change the normal measurements against a solid obstacle? 

A:

Technology used in depth finder, in marine application is mostly like is the best to measure the water from the top.
To measure water level from the bottom of tank one could use a piezo electric ceramic transducer combined with an Analog Front End (AFE) and a micro-controller to measure water level. The diagram below best explains the configuration. 

You can use a piezo electric ceramic transducer from Steminc, TDC1000 AFE from Texas Instrument and a MSP430 micro-controller also from Texas Instrument. There might be other configurations, but currently I am only aware of this configuration.
Piezo Electric Transducer

Analog Front End (AFE)

Click on image for a larger version of the image.
Limitation: The thickness and material of the tank might be an issue. For the most part above configuration work with plastic. 

References: 

Steminc - piezo electric transducer
Piezoelectric matching/backing layer materials
Piezoelectric Modes of Vibration
What is Radial and Thickness Mode Vibration in a Piezo Electric Ceramic Disc Transducer?
TDC1000 Ultrasonic Sensing Analog Front End (AFE) for Level Sensing
How to measure output of the ultrasonic transducer? 

A:

Yes
To answer your specific question, yes, you can use an ultrasonic sensor to measure fluid level. Mahendra's answer describes that. You mentioned that you feel that an ultrasonic sensor is the best solution, but I wanted to add make sure that you were aware of some of the other methods that are used to measure fluid levels.
Measuring Techniques
This list of fluid level measuring techniques comes from an appropriately named website, "A Dozen Ways to Measure Fluid Level and How They Work".

Sight Glass
Floats
Hydrostatic Devices
Bubblers
Load Cells
Strain Gauges
Magnetic Level Gauges
Capacitance Transmitters
Magnetostrictive Level Transmitters
Ultrasonic Level Transmitters
Laser Level Transmitters
Radar Level Transmitters

There is no need to repeat the details of how each of these methods work, but this is a comprehensive list of technologies that could be considered.

