Depending on what tools you have, I can think of two options.
You could replace just the backlight on the screen (backlight is about $10), which would require you to solder a basic flex cable. The water is trapped between the layers on the backlight. Drying it will just leave a permanent mark (if not dried already). Depending on your skill level, I'd say it's a bit tricky to do perfectly. You need to be super careful to peeling off just the backlight panel and none of the lcd part. Also be careful not to crack the lcd.
The more expensive option would be to replace the whole screen. Apple charges 129, but not sure if they'll do it for a water damaged phone.