Sea of Solitude

Maneuver your boat with LS. You’ll come to a gate preventing you from progressing. You cannot miss it. You’re unable to open the gate, at this time.

Turn around and head towards the beginning. It’s approximately half way distance between the gate and the beginning; once there you need to turn your boat around (now you’re facing towards the direction of the gate).

Again, maneuver your boat towards the gate, but this time you need to shoot off a few flares LT while heading back towards the gate (the gate is now open). A black shelled monster emerges from the water, blocking your way. Any time you move towards the monster, a wave pushes you back.

There’s an alley/river path to your left. Take it. Press A to dock and exit your boat. Turn left and walk a few steps, turn left again. Moving ahead there’s a red ladder on a building on your right. Once you are close to ladder, you will automatically climb up to the rooftop.

Walk forward, a bistro is ahead on your right. Shoot a flare LT northwest. This should show you glowing orange entity. Walk towards it. This is your 1st corruption in the game. You will be given a command to clear the corruption. Apparently this command isn’t the same for everyone. I had to use RS while my friend had to use RT. Clearing corruptions crack the monster’s shell. You will have several more corruptions to clear.

I wasn’t really sure what happened here — not even a clue: you see there a ‘girl floating horizontally’

Achievement Corrupted for 15 GS

Just after you meet the monster and dock the boat, this is on top of a building.