How do I repair an iPad with water damage?

If the logic board is damaged and unrepairable, there won’t be any way to retrieve the data. The information is encrypted on the memory chip in current iOS devices, so even removing the chip (which would require specialized equipment anyhow) wouldn’t allow you to read it.

Your best bet is to disassemble the device and clean the parts according to a something like iFixit’s Electronics Water Damage guide, and hope you can bring it back to life. Otherwise, a specialized company like microsoldering.com may be able to repair any severe liquid damage to individual components.