Just plopping the phone in an ultrasonic cleaner isn't going to help. It's better than putting it in rice but if you have access to an UC, leverage it to it's maximum effect.
Remove the logic board and remove the shields so that you can see what parts of the board were attacked by water/corrosion. This will help you with your troubleshooting later on. Then clean it in the UC. You really shouldn't put IPA in the bath though as it has a low flash point and the UC will really excite the molecules making for a hazardous situation. Ideally, you should use a cleaning solution like Branson EC and then rinse in IPA.
Once that is done, then start probing the board, specifically, check the PP_VCC_MAIN & _PP_BATT_VCC to see if they are shorted. Only then would I recommend putting a battery on. If it doesn't boot, then you have to dig deeper in the circuitry to understand what's going on.