Since you don't say just which iMac this is and what version of Mac OS X, if it has 10.6 on it, do you have the install disks and can you try booting, holding the C, and see if it will start from the install disk? The beep sequence says it thinks it was interrupted during an update of the firmware from a CD, so maybe it will start from the install disk.
If it is Lion, try starting holding the Command and R keys so it boots from the recovery partition.
If neither of those work, it might be a good idea to take it to an Apple genius bar and have the techs check it out and see what is going wrong.
Liquids and electronics never play nice together.