SeaLaunch experienced a failure of it’s Zenit-3SL rocket on February 1st 2013. Early indications are that the  RD-171M main engine engine shut down approximately 20 seconds after launch. A failure review board is being formed and according the the Sea-Launch website they can confirm the following.

• The Zenit-3SL successfully completed all pre-launch processing activities, with all systems and environments within requirements up to the on time liftoff at 6:54:59 UTC.
• At approximately 11.4 seconds into flight, the Zenit flight control system detected an exceedance of a pre-programmed roll limit and responded appropriately with activation of the on-board thrust termination sequence, which is designed to ensure a safe outcome in the event of vehicle loss of control.
• In accordance with the flight termination logic, RD-171M main engine thrust was terminated 20 seconds into flight, resulting in impact of the Zenit-3SL with the IS-27 spacecraft with the Pacific Ocean surface approximately 4 km from the Odyssey Launch Platform.
• Nobody was injured and the Sea Launch vessels were not damaged in the incident. A search effort conducted this morning from the Sea Launch Commander-based helicopter identified no recoverable debris.

The SeaLaunch rockets are different than land based rockets in the way they handle failures and terminations of flight. Land based rockets have an on board explosive charge which destroys the vehicle in flight to protect people and property on the ground. SeaLaunch only needs to protect the launch platform and staff due to the fact they launch from the middle of the ocean. Therefore when a failure is detected the engine will be allowed fire for 20 seconds in order to get the rocket far enough from the launch platform to prevent damage. The engine is simply shut down as you can see in the video below. There is no explosion of the rocket, it simply impacts the water some distance from where it launched.