Simple question:  Can you hook up a locator that draws 12 volts to a 12/24 volts system and operate it while you are running the 24 volts system?

You can't run a 12v anything on 24 volts but you can hook it up to one battery. If you have 2 TM batteries and a cranking battery for your engine hook your sonar to the cranking battery.

12/24 volt TM's are just that, 12 volt and 24 volt.  As long as you use the 12volt wire going to the TM you can use it for a 12volt source.  With that said, it's not recommended you use the TM batteries to run your sonar.  In your case, with a 12/24 volt motor is not really a problem as with mosfet controlled motors but some TM 's have a high frequency feedback that can damage the sonar.  

Even on a straight 24 volt system with the batteries connected in series, as long as you connect the negative and positive to one battery, it's still only going to be 12V.