I thought I'd take a stab at your Post since I encountered this same situation about 4 years ago.
My neighbor had built a pond in his yard and took the dug-out dirt and built a berm on his side of the fence. In doing that, however, water that normally flowed from the back of his yard to the back of my yard NOW got diverted to my side yard. WIth that, it started to flood one of my basement window wells. I did regrade but it wasn't enough.
I did ask him to move the dirt which he did. I even took pictures so I could back up my position. I took them from the 2nd story window on the side of the house (while looking down) so he could actually see where the water was flowing. I needed more than just my 'word' to show him the problem.
To boot, as it turns out, in MY Municipality, it's illegal for a homeowner to alter the natural flow of water off of his property where it would have an 'adverse affect' on a down-grade property. (The adverse affect may be on a lot NOT necessarily 'bordering' his property, you see.) Sooooo, if he didn't move his berm, there was legal 'relief'.
You could try to talk to your neighbor first. BUT, before you do, I'd check w/your Municipality to see what they can tell you about the laws in your Municipality. Just tell them you're curious, for now. (Don't tell them there's a problem.) If they say that a neighbor can't do as I described in MY Municipality, then if your neighbor doesn't budge on his position, you can always tell him that he's breaking the law. The law's purpose is to provide relief to neighbor's like you. (Have him change boats, if you would. Let him see it from your side of the fence.) BUT, remember this - You'll have to live next to this neighbor for some time to come. Be prepared for any potential fall-out from this encounter if you end up resorting to quoting the law.