Dropped in water, won't boot.

Your battery reading 100% at all times MAY be a bad battery. if it is shorted inside, it will ALWAYS read 100%.

Water Itself is not damaging to the board. water in it's purest form is non-conductive. the actual problem is the minerals in the water. (Iron, Calcium, Sodium, Lime) These are conductive and cause damage. unfortunately, they have a much higher boiling temperature than water, so when the water evaporates, these minerals are left on the motherboard. Rice doesnt actually help the damage, but rather speeds up the drying porcess. If the phone functions WITH the rice treatment, it would have worked without the rice treatment.

as a water damage repair, it still is only 50/50 as to whether or not you can get the device up and running. follow the next steps, andyou may have a functional device.

1) Isopropyl: Take a 99% rubbing alcohal, also known as Isopropyl, and a toothbrush. (New, as it can not be previously contaminated by conductive materials such as saliva, or tap water) soak the board in the isopropyl, and scrub diligently. make sure not to be terribly rough on the board, but still use a heavy handr

Check to see if board works

2) DeOxit (Electronics cleaner): I have used many brands, but DeOxit is the the one i have had the most success for, so i suggest this brand. others may work, and it is only a personal preference. spray the cleaner onto the board, and seal in a tupperware container. let sit for an hour, then re-soak with isopropyl to dilute any lifted dirts, and scrub with the tooth brush. Essentially, after the DeOxit has sat, Repeat step one.

Check to see if board works

3) Sonic Wash: This is implying you have access to the tool required. usually need a repair shop environment to have one The sonic wash is a tool, usually cosing between 300$-500$. fill it with distilled water and a liquid electronics cleaning solution. this pulses the cleaner underneath components where a brush can not reach. though this is the most successful procedure, it is also the most expensive of the three.

i hope this helps, and if it does, please accept the answer.

4) After all of these procedures, if any funtions aren't working, try replacing the modular component that the function is related to. (Antenna's for signal, Charge port for charging, Camera for camera function, ETC.)

Anything further than this will be a board level failure.

My previous answer is the Do's for water damage. this next response is personalized for your issue specifically.

if your battery is reading 100% at all times MAY be a bad battery. if it is shorted inside, it will ALWAYS read 100%.

the power button on the S6 models is pressure seated. meaning the screws need to be in place to function. if the alignment of the button flex, is not aligned with the board, then you may have a disconnect. to test this, remove the back housing, (with the power button) and short out the two pins that the flex connects to on the mother board for 3-5 seconds. if your device boots, there is no problem with the board. if it does not, then the board is having issues.

The power button may be a cold circuit. use the continuity setting on a multimeter, and hit the pins on the power button flex with the two leads. If done correctly, when you press the power button in, the multimeter should beep. if it does not beep, then the power button needs to be replaced.

There might be some issues after you replaced the charging board, just to be save, get a sealable container, fill it with rice, and sit your phone in it, and seal it. Check back in 4-6 hours just to make sure all water is gone.