old water damaged iPhone with power issues - anyway to recover data?

probably not. once the phone innards get wet, your worst enemy is time. the quicker you can get it dried, the better. the longer the liquid sits, especially in a closed phone, the greater the damage from corrosion.

here's what i recommend. get the motherboard out and give it a good 1+ day soak in 99%+ isopropyl alcohol (IPA). if you see any visible residue or corrosion, give it a very gentle scrub with a very soft tooth brush (like the kind dentists give free to young kids). after the scrub, give it another soak (at least half a day this time) in clean IPA. agitate a little then take it out and give it one final rinse with clean IPA. bend a paperclip open and pop one end through an open hole so you can hang it to dry in a slightly (or much) warmer room than usual room temperature. make sure the humidity is as low or lower than normal (ie, using a hot shower to raise the air temperature of the bathroom would defeat the purpose - i know this should be obvious, but sometimes ya just gotta say it). let it air dry for at least a day or two.

at this point, if you have known working and dry parts, you can reassemble and give it a shot. i'd be hesitant to use the original parts because of both the liquid and physical damage.

at this point it's a crap shoot. if the board is dead, no amount of IPA will resurrect it. but i have used this method to bring motherboards back to life than had suffered major liquid damage.

good luck to you