My phone went swimming, how can I power it up to back it up?

@carl30665 not sure what you mean by "bypass the battery". If your phone get water damaged it is most likely more than just a battery issue. You will have to deal with the water damage first in order for it to power on.

For now you want to stop continuing to try to charge, sync or otherwise use your phone. This potentially can make a bad situation worse.This may further damage your phone.

The next thing to do is to clean it. Disassemble your phone using something like this video. You must remove all EMI shields from the circuit board. If you do not remove those, you might as well not do anything. After you removed those, clean the whole board, connectors and all with +90% isopropyl alcohol. Follow this guide , and even so it was written for a 3G all the points are still pertinent to your phone. While you clean your board, check for any obvious damage, like burned or missing components etc. For anything odd looking, you can post images of that with your QUESTION. Adding images to an existing question That way we can see what you see and hopefully guide you further

When it is properly cleaned, replace the battery. All these steps are to avoid delayed failure caused by corrosion. Best thing to do would be to clean it with an ultrasonic cleaner, but if you do not have access to one, this will at least help. Once all this is done, reassemble your phone and reevaluate. Until it is cleaned, everything will only be a guess. Even with all this, water damage has only a small chance to be fully repaired, but at least it should allow you the opportunity to get the files backed up.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.