One thing to consider is what credit card you used to purchase the computer originally. Many credit cards (American Express, for example) include accident protection for purchased items, which may cover the cost of an Apple Store repair. It's worth checking to see what coverage your card gave you.
I'm kind of a nut for preserving whatever warranty you have. You paid Apple for that warranty (if you got the AppleCare extended 3-year warranty, you paid a lot for it), and it's worth getting what you paid for if you can. Even though Apple doesn't warranty "accidents", Apple will often keep the warranty open if the accident is repaired by the Apple Store or by an Apple Authorized service provider at your expense.
If you have an Apple Store handy, it's worth trying to take it in to the Genius Bar even though they may void the warranty. They're employees of a big rich company, and they know their system, so they know how to turn it to their advantage. Sometimes they'll feel sorry for you, and give you things you're not legally entitled to. In any case, if Apple does the repair for you, your warranty will be preserved, and the work performed will carry a 90-day warranty even if it's out of your basic warranty period. Since Apple has a ready supply of tested parts, they may be able to do the fix cheaper than you can. At least get an estimate; if you don't ask, you don't get.
But if you want to do the work yourself, and you're prepared to give your warranty up for lost, here are some places to start:
Electronics Water Damage (applies to any kind of liquid spill)
MBP Retina 13" Early 2015 Guides
The usual advice in these cases is to:
- disassemble the system
- wash all the circuit boards (logic board, I/O board, MagSafe DC-in board, SSD, maybe the Airport/BT card) in purified alcohol or distilled water
- dry completely
- reassemble (being careful to apply the heatsink compound correctly to the logic board and heatsink)
- replace the battery
- charge the replaced battery overnight
- attempt a restart
My guess is that when you disassemble the computer, you'll find that other liquid sensors have also been hit. This will give you a better sense of the range of the water's impact.
In general, a liquid spill typically harms the battery; replacing the battery is a pretty standard thing. Since your MBP was closed inside your backpack, it's possible that your keyboard is unaffected. Once you're able to start the computer again, it's a good idea to open up a text document and check each key (shifted and unshifted) to make sure they work correctly. Otherwise, you may need to replace the keyboard and/or top case.
MacBook Pro 13" Retina Display Early 2015 MagSafe DC-In Board Replacement
MacBook Pro 13" Retina Display Early 2015 I/O Board Replacement
MacBook Pro 13" Retina Display Early 2015 Upper Case Replacement
MacBook Pro 13" Retina Display Early 2015 Logic Board Replacement