underwater robot, using arduino

[omkar]

hello,

i'm designing a robot(AUV). i need a sensor, and a gps module for underwater application.

i can find so many gps modules, but which one can work inside the water?

i need a sensor for finding obstacles, inside the water.

friends, plz suggest me.
this is my final year project.
i'm using arduino.

[adafruit_support_bill]

GPS signals are too weak to go through water. GPS systems designed for divers use a floating antenna on a tether.