Ultrastick,
The problem you are describing sounds more like a transducer failure versus software. If you can find someone else with a Humminbird DI model unit you could test your 570 DI on their boat to see if the problem is in your unit or the transducer.
The cord you are referring to would be either the AS-PC2 or the AS-PC3 (depending on whether you need a DB9 Serial Port connector or a USB connector for your computer) and would allow you to update the software and manage your waypoints via the HumminbirdPC program. I would wait on getting this cable.