Do not walk into moving water. Fifteen centimetres of moving water takes an adult off their feet. Do not drive into it: sixty centimetres floats most cars, and a floating car goes where the water goes. The instruction published by civil protection agencies is deliberately blunt — turn around, do not drown.
Flood water is not water. It carries sewage, fuel, chemicals, sharp debris and, where cables are down, current. Skin contact is a health risk and open wounds must be covered before any contact at all.
Before, if there is warning. Move upward, not outward, if roads are already affected. Take medication, documents, a phone and a charger, warm dry clothing. Switch off gas and electricity at the mains if you can reach them safely. Move what matters upstairs; move vehicles to higher ground.
During. Higher floor, not the roof void unless it has a way out — people drown in loft spaces with no exit. Keep the phone charged and keep it dry. Signal to rescuers from a window. Do not swim.
After. Do not return until the authorities say the building is safe. Assume electrical systems that have been under water are unsafe. Assume the water supply is contaminated until told otherwise. Photograph damage before clearing it, for insurance and for the record.
Source. WHO, Flooding and communicable diseases and WHO emergencies guidance, https://www.who.int/emergencies ; flood safety instructions as published by national civil protection agencies and by the US Federal Emergency Management Agency at ready.gov, cited rather than fetched because that server refuses automated requests from this platform.