The best thing to do is put the phone in a warm area for a few days to let it dry out, Unfortunatley Sony Ericsson will not honor any warranty on mobiles that have suffered water damage so your warranty will be shot now, After a few days of drying out, try to power your phone back on, Hopefully you may see the screen light up and display information. If the screen displays wierd symbols or distorted images it could either be a damaged screen, damaged firmware or worse damaged hardware, a damaged screen will set you back about £20 -£30 depending on the model of phone, the firmware I personally do repairs to for a fixed fee of £5, if its the hardware then its realy time to get a claim on the insurance or buy a new phone.
If you want more information contact myself by email at drewishuk@yahoo.com
Andy
May 2008