I'm having problem with VS2013 watches. For example a watch that adds two floats is ok but adding a double and a float is not. Why is that and what should be done?
This is a game project using Unity3d and the .Net version is 3.5.

I recommend you to use VS studio 2015 since unity recommends that version integrated with the vs studio tools for unity where there is a lot of advanced debugging tools, just like rendering debug and everything is simplified for that version.
There is a tutorial about how to install that tools and connect them:
https://youtu.be/lKHCrRNTf_s?list=PLReL099Y5nRfseAg0k1SJOlpqdcsDs8Em