I have the DJI Mavic Pro. Today I was attempting to fly over water.  It started to fly and didn't get far from me and it started to decend into the water.  I hit the return home key and lost signal went straight into the water.  What do I do?  

I'm sorry to hear that.

First, please upload the flight log (a TXT file stored on your mobile device) using these instructions and then post a link to the uploaded log in this thread.

Also, if it's possible to safely retrieve the Mavic from the water then you should try to do so, as that will greatly increase the likelihood of determining what caused this to occur and may also decrease the financial impact on you. By any chance did you purchase DJI's Care Refresh before this happened? If so, the importance of getting it back is significantly higher because if you can retrieve it then you're looking at paying $79 to get a replacement instead of $750.

show flight with stick input and record the video playback, post here... your information doesn' say anything.

I regret the trouble you’ve been inflicted and  feel sorry for your loss. Kind DroneFlying will help to check the flight data, on the other hand, please contact our support to start a ticket, we will help to make data analysis too, if it's malfunction, we will compensate your drone according to After-Sales Policy.