I have been scammed on ebay. What do I do?

I recently purchased a brand new unlocked iphone 5 for 599.99. The listing said that there was a 14 day money back guarantee so I felt safe about the buy. However, 3 days later when the package arrived to my door, I was disappointed to find that the only contents of the box was a 3/4 full opened bottle of water.

I've opened a case with Ebay and Paypal. I also filed a case with American Express.
They said I wouldn't be charged for this purchase until the investigation ends, but how can I prove that I received a bottle of water in place of a 600$ purchase? The only thing I could think of was to take a picture as soon as I opened it.

What do I do?

You pretty much did all you could do

The seller loses, make sure you give the bad feedback to them, I'm guessing they had zero feedback already or bad feedback… So add to it. Surprising as they must not have realized they won't get to keep the money.

This is awful for you.eBay sure isn't the place it used to be! I've been buying on eBay over 10 years. Before "buyer protection" or PayPal, yet I never had a bad experience back then. Now all the scammers are into it. Sadly buying a phone, camera, computer or game console or any new "brand" item is fraught with danger of being scammed or receiving a fake product. Hopefully you will win your case but the seller has really been deceitful, sending you a "weighted" parcel registered mail making it a case of "who is lying". The seller has "proof" he sent a parcel that you got, but now you are saying it was a bottle of water! Who is lying? Who has the better buying or selling record? As if you are going to video every parcel opening (and that doesn't prove much as maybe you opened and re sealed it - says the seller). My only advice is to not buy such items on eBay, no matter how good the price. For $600 you need guaranteed product and quality so that means spending a bit more and going to a real store. I hope it all works out ok for you. If the seller has done this before you'll be ok, but if you're his first victim and he has high feedback and you have low feedback I suspect you might lose out.

I have seen this type of thing on JUDGE JUDY Id file a small claims court complaint then find judge judy to hear the case, SERIOUSLY id also file a complaint against him on ebay and paypal for a full refund and turn him into the local police where ever he is located and you have his return address on the package

yes take pictures of it all thats your only evidence