iPhone turns on but you the screen is completely dead?

Chances are the water damaged your FPC connectors or the LCD/Backlight circuit.

Do the following as soon as possible:

1. Open your phone and remove the logic board (follow this guide)
2. Inspect the logic board, especially around the connectors and look for corrosion.
3. Inspect both sides of the board. Unfortunately, 80% of the board is covered in shields. That's usually where the damage is occurring.
4. Put your board in a container with >90% isopropyl alcohol and let it sit for a while.
5. Use a soft brush, like a toothbrush and lightly brush away any corrosion you see.
6. Rinse in alcohol and repeat.
7. Let it air dry for a day.
8. Re-assemble and hope for the best.

A professional repair shop that does water damage repair may be able to recover your phone or the data because they have access to pro-level ultrasonic baths and specialized cleaners as well as the skills to troubleshoot your board.