what's the best Solution when a mobile drops in the water?

@yousef200727 just like any other device, the very first thing you want to (not) do is to try to turn it on. Remove the battery as quickly as possible. Then you want to disassemble the device completely. Clean every part as thoroughly as possible with high grade (90%+) isopropyl alcohol and a soft brush. That must include all connectors. Depending on the device itself, you must remove all EMI shields for proper cleaning. For a general idea check this guide. Once it is properly cleaned, you reassemble that device and use a new battery. The evaluate for any failure and repair as needed. Water damage is the hardest to diagnose and to repair since virtually any part may have failed. Yes, ultrasonic cleaning would be the best way to go about it, but most of us do not have that equipment in our possession.