I got 29 hours, 40 Minutes and 18 seconds out of my 9Ah battery to be exact.  Real world results might vary, as this was just a static "on" test with the transducer in a bucket. I can turn the unit off when not in use or turn the backlight down. Just the GPS cannot be turned off, but I can turn off the transducer, though I'm not sure I will bother.  I believe the screen itself uses the most power, dimming the backlight to 6 or 7 should help save juice.  My test was run at a full, level 10, backlight

For testing I used one of those Chinese battery load testers. 
  (You need to Login or Register I was able to plug the fish finder in and use that instead of the artificial load generated by the board.  The board logs the time a load is drawn and the Ah. It works slick for the price, plus I can test batteries with it.

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Here is my rig:
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It consists of a thin cutting board with a lock-n-lock container for dry battery storage, zip tied down with a inverted plastic electrical box sandwiched between a Ram mount and the cutting board.  The electrical box houses a switch and a bright blue LED, for power on status, and serves as a riser allowing me to fold the screen flat against the battery box for transport.  The yellow cord is a 3 foot extension, allowing me to mount the unit to my thwart and have the battery elsewhere. 

On the business end of the battery I have a 3D printed sleeve that connects to the battery terminals.  Wired to that is a 2A breaker in place of a fuse and a tiny buck converter to bring the voltage of the Ryobi battery down to 14.5V.  This is all wrapped tightly in electrical tape, not pretty but it works.  Even though the Ryobi system is 18V, a fully charged battery is over 20 volts, and I did not feel comfortable feeding that to my unit.  14.5V sounded more reasonable and there is very little heat generated by the buck converter at this voltage differential. 

One other modification I made to the unit is that I ditched the Lowrance power connector and soldered .062" pin connectors to a LED power cable. Only two pins are used for power, the rest are for NMEA, which almost no one uses, and certainly not in a canoe.  The two larger pins are the power pins, and the black negative pin is located between the two plastic notches. I will be installing some heat shrink to protect the connection from the elements a bit. It may be hard to see in the pic, but I have a small zip tie securing the wire to the threads of the connector so the wire cannot pull loose.
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