It would certainly stop measuring heart beat and probably steps as well. I think this comes down to what is making the police so confident in their hypothesis, and what you've described would probably result in steps and heart stopping at the same time and that throws up various scenarios and I don't see them being so confident. However, let's say the data showed she took her last step at 9.20 and her (please let this not be the case) last breath at 9.30 then this points to her being in the water sadly. How she got there is anybodys guess though, I'm not buying the falling in for various reasons, I think she was assaulted by someone who may then have removed her somehow.