To me, this sounds like a bad display connection. It could just be the cable on the screen, or it could be the connector on the logic board. I’ve found lots of times that even when something doesn’t appear damaged at first, parts can just go bad no matter what. You could try purchasing a new display, or making sure the little bits of the connector on the logic board aren’t damaged (with a magnifying glass)
First, I would run through the basics questions… Do all the flex cables have a good connection? Could it possibly just be a bad part? If none of the little things provide any answers, I would take a closer look at the connecters on the motherboard with a magnifying glass. Do any of the little pieces look bent or moved? The display and home button is a good sign, but you may want to go back and double check everything went smoothly.
Water damage is always tricky, and unfortunately there usually isn’t a simple answer. If the rest of the phone is working, I would say that replacing the charging port on the logic board may fix your problem. I would open the phone up to get a good look at the damage. If it’s clear there’s been lots of shorts and you can see burn like marks, I’d say the phone is toast. If not, give soldering on a new charging port a try.