An autonomous underwater vehicle (AUV) is a robot which travels underwater without requiring input from an operator.AUVs carry sensors to navigate autonomously and map features of the ocean. Typical sensors include compasses, depth sensors, side scan and other sonars, magnetometers, thermistors and conductivity probes.

University of Florida robotics engineering students developed a robot Subjugator , which can to swim through underwater tubes without a remote control.

Uses :  

These vehicles are used in underwater research because of their ability to work autonomously. They do not require guidance from humans despite having a capability to swim in dangerous underwater.They can collect data and material from deep inside the sea  which can later be used in various research activities.

Useful Link :
Lego AUV 

Related Book :
Autonomous Underwater Vehicles: Modeling, Control Design and Simulation(Buy from Amazon)
Project Report on AUR

Things required to build this project :
Lego Mindstorms Kit:   $200
Pelican Case:   $7
Propellers: $3
Waterproofing Supplies:   $2  
TOTAL     $212

Design and fabrication of Submarine: