Did you make sure to use the right battery and replaced the cover properly? If so and still doesn't work, then call Marcum may have to warranty it.
From the manual:
Remove the battery door located on the lower backside of the unit. Insert a coin into the coin slot and turncounter clockwise. Place the coin under one of the slotted corners and apply gentle upward pressure untilbattery door opens.
Place lithium CR-P2 battery into the molded battery compartment. Replace batterydoor by applying gentle downward pressure with your finger while turning the coin slot clockwise until you feel a gentle click.
You are now ready to use the LX-i to obtain depth readings. The battery will lastroughly twenty hours of continuous run time or multiple years worth of depth readings
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"My worry is my wife (when I'm dead) will sell my fishing gear for what I said I paid for it"... 
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