Hi Blane sorry to hear about your HTC m8. As far as HTC's warranty goes, it sounds like water damage is not covered unless you have their Uh Oh Protection which you may have or be eligible for depending on when you purchased your phone. Look here for more info on Uh Oh Protection. http://www.htc.com/us/uh-oh-protection/
Is the water damage indicator on your M8 white or red? If you are not sure where the water damage indicator is then look here. https://support.t-mobile.com/docs/DOC-10... I know this is for t-mobile but it is in the same location for other carriers as well. I also own an M8 that is not from t-mobile and this is where my water damage indicator is on it. Red indicates water damage and white should mean no water damage. If the sticker is white you could try a warranty claim and hope for the best, but if they open the device and see evidence of water damage even though the indicator is still white more than likely they will reject the claim.
I received my M8 from my niece after she had cracked the screen and had water damage to the phone. She thought is was unrepairable and bought an iPhone 6, so that is how I got a hold of the phone. The phone would not power up at all and the only sign of life was the LED charge light when connected to a charger. What I thought was weird is that her water damage indicator was still white when looking at it through the flash lenses. So it made me wonder if there was actually any water damage. So I used the guides on this site to disassemble the phone and check for problems. I noticed mineral build up around the vibrator motors connection, around the battery, and there was some present on the LCD's ribbon cable as well. So you can't always trust the water damage indicator. She told me she tried turning the device on after it got wet, which is not good. Upon further inspection I removed the battery and checked its voltage. The voltage read at 2.75v and even after trying to charge the battery I would not get any higher than 2.8v which told me the battery was damaged by contact with water. I know a phone won't power on unless its voltage is at least above 3.4v. So I ordered a replacement battery. I also noticed a short in the LCD ribbon cable due to a small burn mark. I then ordered a new display assembly too.
The most important thing I suggest you do if your phone is not covered by warranty is to disassemble the phone, remove the battery and check for any type of corrosion or mineral build up on the motherboard and other parts of the phone. If present you should clean the motherboard and parts with 90% or higher Isopropyl alcohol and a small soft bristle brush(clean toothbrush works well). This helps remove any minerals left behind by water that can lead to corrosion or shorting of components. Your front cameras connection may have corrosion present which may lead to the camera not functioning. Once you have cleaned the motherboard and other parts well with the isopropyl alcohol you can reassemble to see if the issues are still present. If the camera is still not working and its connection to the logic board is clean and undamaged then you may need to replace the camera. Sorry for the long story but I thought you should be aware that cleaning with isopropyl alcohol can help revive a device that seems unrepairable, and also keep corrosion from spreading and slowly damaging other components in a phone.
After cleaning my M8 with isopropyl alcohol and installing a new battery and display I have not had a single issue with it. If you need an idea of how to clean your phone with isopropyl alcohol, you can find many instructional videos on Youtube and I believe there are some guides for that on this site as well. Best of luck with your M8.