Robots are going underwater and scientists from Technological Educational Institute of Crete proved this. They created an autonomous robot with fins.

Via Atmel: 

Each fin is comprised of three individually actuated fin rays, which are interconnected by an elastic membrane. An Arduino Mega (ATmega2560) at its core runs custom real-time firmware that implements two Central Pattern Generator (CPG) networks to produce the undulatory motion profile for the robot’s fins, through which propulsion is achieved.

The prototype, which is fully untethered and energetically autonomous, also integrates an IMU/AHRS for navigation purposes, a Bluetooth module for wireless communication and a camera to capture underwater video. This footage includes experiments conducted in a lab’s test tank to investigate closed loop motion control strategies, as well as clips from actual sea trials. The robot is powered by a 7.4V LiPo battery

Not long ago we wrote about The Row-bot, which is also an autonomous swimming robot, but it swims on the water surface.