Where did you find the amp draw? The lowrance spec sheet for the Elit 5 and the mark 5 units is 0.25amps>
http://www.lowrance.com/Root/Lowranc...0157-001_w.pdf
Everything get complicated with research. People will run there 12volt battery down to 11 volts, charge it up and run it the next day. Will it work....yes...will it degrade you battery and shorten the lifespan considerably....yes.
I just come from the camp..."if you are going to do something...do it right" the first time, (if you can).
Your bait tank pump should have the amp hour draw as well written on it. Do the math, stick around 60% draw and you will be fine for years to come.