I came across this website a while back which lists the power draw of some of the older legacy Humminbird fish finders. It might be useful in selecting a suitable replacement:
Legacy Humminbird Fish Finder - Power Draw Assuming you are looking for a B&W model to keep power draw to a minimum, you might also consider a Lowrance/Eagle model. X-4 and X-4 Pro were the last B&W models produced by Lowrance, but Lowrance produced a multitude of models that were very similar going back to their Eagle Cuda models: Eagle Cuda 300, Lowrance X50DS, Eagle Cuda 242. These units all have the same 240x160 resolution and similar if not identical specs. There were also several lower resolution models(X-37, X-47,Cuda 168,Cuda 128+more), but they all use the same transducer(HST-WSU). (These lower resolution units make good donar units for a spare transducer for the right price.)
Lowrance lists the power draw on these units at 170ma with the backlight off. This is a bit more than latest made Humminbird B&W models(230, 175), but still low power. Also, I am not sure if those are apples to apples comparisons on power consumption as they may have been tested differently.
As boats are traded and subsequently upgraded, these B&W units are often removed for a fancier color units with newer technology. Fortunately for us canoe fishermen, these units often end up on the used market. I have found there are typically two types of sellers. One seller lists the item at full retail or near retail price and you will see a lot of these types of listings. The other seller is one that just wants to get rid of the item and hopes to get a little bit for it. Obviously these sellers actually sell their items fairly quickly, so their listing won't last long. Be patient, but also be ready to message a seller if the unit you are looking for comes up for a good price.
Sometimes you will find a unit listed as just the head unit without the transducer. Often times you can pick these up for cheap. The catch of course is that you have to find a matching transducer, so this strategy may or may not save you money in the end.
Keep your eyes peeled, and good luck in your search!