What would happen if water get inside the device?

Even if the water completely evaporates, any residue that was in the water could remain behind, and as Allan Eckert has mentioned, cause a short circuit which could further damage the phone. Do NOT fall for Internet "cures" of placing the phone in rice or any other type of desiccant as these will NOT remove the residue. Apple will not service this device. At best they would offer a replacement, typically at a cost. What the cost would be would be dependent on, whether or not, you elected to purchase an AppleCare+ plan with this phone.

If it doesn’t short out the whole device, or one or more essential components then corrosion will eventually ruin it.

The fact it won’t power on at all indicates it short circuited and fried the device, likely as soon as it went into the water.

The only real fix for water damaged electronics is to replace them.

Electronics and water don't mix. The water usually shorts out the components.

Sounds like it is time for you to call your local Apple Store and make an appointment for them to examine it. Hopefully afterwards they will offer to sell you an out of warranty replacement for it.