My iPhone 4 lock button broke, will Apple fix/replace it for free under the one year warranty?

I've had my iPhone 4 for just under a year now and a couple weeks ago the lock button broke. At first, it was just difficult to press down and now it's just completely stuck. I've been using the assistive touch but it's kind of a hassle. I still have a few days left of my one year warranty, so if I went to the Apple Store would they fix/replace my phone for free or would I have to pay? Also, I was having issues with my speakers previously so I went to get it fixed and they said my phone has water damage and I would need to pay to fix it. Will the water damage effect them fixing my lock button too or is it different? Any advice/guidence is greatly appreciated!