What should be my first step?

I would venture to guess that it will be dry as a bone after a year...and like they say, "time heals all", so I don't think you would have caused it more harm by leaving it sit for a year. so:

Have you tried plugging it into your computer to see if it is recognized?

Have you left it plugged into power for a few minutes and then tried to see if the vibrate mechanism works (switch mute toggle on/off)?

Try holding the home button while plugged into power and see if you hear voice control come on.

Could be that the LCD is dead and you are not getting any visual signs of life, but other audible signs could be there. I would also guess your battery will need replaced.

If it were me, considering it is water damaged, I would have it open and would pull the logic board and clean any signs of corrosion with alcohol and a toothbrush. I would also swap out the battery while I was in there. I would then move on from there w/ my troubleshooting.