My iphone is water damaged, but fully works when connected to a power source?

Not just that the logo comes up, but the phone fully functions… Is there any way to fix it myself?

Plunge it into a box of rice and leave it 12 hours, all the water will be drawn out, there can't be much wrong with it, just a drop of water in a place it shouldn't be.
Advice I was given by an apple shop.

Consider contacting your local Craigslist iPhone fixer to look at it for the battery might… Wait, lol yea just buy a battery, take off the back plate (5 seconds to do this) and pop out the old one and place in the new. Battery's cost like 5$ on EBay.

Water damage can be a hard thing to fix, especially if you don't want to take your phone apart.

Both void any type of warranty, so ultimately its up to you.

I've had really good results stopping water damage by sealing the phone in a air tight container with instant rice for more than 24 hours.

The rice pulls and absorbs all the moisture out the surrounding area. So the smaller the container; the larger amount of rice; and the longer it sits the better.

Hope this helps