How much does it cost to repair iPhone water damage?

As LD150 has mentioned any type of liquid damage is not covered under the Apple 1-year warranty, but if you elected to get an AppleCare+ plan, and that plan is still active, you may find that you can get a replacement phone at a reduced cost.

Regardless, see if the following support articles can help:

• iPhone Repair and Service - Apple Support
• Water and other liquid damage to iPhone or iPod isn't covered by warranty - Apple Support

Water damage cannot be repaired by Apple.

So it's a new phone, but you may get a replacement from Apple Store at reduced price if you take it in for assessment.