What is the standard warranty period for the high-voltage battery?

The standard warranty period for the high-voltage battery of Audi e-tron is 8 years / 160,000 km.
Are Batteries safe after an accident & can water get in?

The battery is designed with crash safety in mind. The battery concept provides a level of safety that goes beyond statutory requirements. The underbody bash plate protects the battery from any road undulations.

The battery is designed to be very well protected by the underbody guard and the battery housing. A solid protective frame and a combination of aluminum profiles and cast components ensure a high level of crash safety.

The protective coating and seal are designed to strictly fulfill leak-tightness requirements. The battery system is also designed to provide resistance to corrosion and environmental factors.
Are people or the environment at risk in any way if the battery is underwater?

No; providing that a high-voltage battery is not damaged, the battery system is sealed completely and strictly fulfills leak-tightness requirements.