Just because your batteries are new, that doesn't mean much.
They may be new to you, but you don't know how long they were on a store shelf,
and you don't know if they were kept in a fully charged state.
Also, just because you have 13.6 volts, it doesn't mean that your batteries
can sustain any kind of a load. It just means that they are fully charged.
Fully charge and load test your batteries.
Re-check all electrical connections, and physical connections at the back of the unit.
You may try bending the electrical connectors on the hot power wire out a little for a better
connection (DO NOT BEND BOTH CONNECTORS AT THE SMAME TIME OR YOU WILL BLOW A FUSE).
If your sonar is sharing a battery with something else, then get yourself a small
marine battery just for the sonar. You may have it hooked to the same battery
as your main engine. As Ddorriety said, cranking up the main engine may have pulled the voltage
down so low, that it caused the sonar to stop working.
I'd check the above things first, then worry about the software version if
you still have a problem.
Good Luck.