Electric water heater power switch
Q1: Should there be an (emergency) power switch next to an electric water heater?


Doesn't have to be, but we always have a disconnect switch by water heaters for any commercial building.


  Q2: If A1 is no, is it okay to have one anyway?


Yes, it's fine as long as it's wired right and such.


  Q3: If A2 is yes, what kind of power switch should I use? I don't think I can use a regular light switch because this cable is 10/2 AWG and both the black and the white are hot.


Seems you've already discovered it, but yes they do make double pole switches. Typically a disconnect is used (as answered prior) and they're not any more complicated, but are safer. It's just a big switch.