What is the voltage telling me on my fishfinder? At what level (high/low) should I be concern?

I have heard various things over the years but never sat down and actually learned how to understand what it really means...

To operate a Fish Finder it Takes Voltage and Current. If you look at electricity like fluids you have Pressure and Volume. Voltage is the Pressure you can get that thru all sizes of wires. Current is like Volume you can't get the same volume of water in a 1/4" hose as you can a 2" hose. Same thing with current you can't get the same volume of current in a 30 Gauge Wire as a 18 gauge wire. So if you were filling up a swimming pool a 1/4" hose would not supply much water same thing goes for a Fish Finder it needs voltage but also the proper current so larger diameter multistrand copper wire can keep voltage steady and supply more current so the unit has the proper amount to operate all the time. Then you can throw in things that restrict current like corrosion in the wires, resistance etc. Electricity warms the wire as it passes through and over time came build up resistance in the wiring. Using a larger gauge than recommended will allow for the resistance to build up and not have a negative effect on providing adequate power.

Hope this helps.

Thank you.

What is the actual voltage read out mean? 11.8v vs say 12.1 vs. 12.9.....

A fully charged battery in good condition has 12.6-12.8 volts.
If you are not getting that on your voltage readout, there is resistance in the wiring/connections that is causing that voltage drop. As the battery voltage drops during use expecially when operating pumps, lights, electronics, frequent starting, etc--that resistance will get voltge below what will support the sonar unit.

To put it more simply, in terms of your question:
Most fishfinders operate reliably in a range from 10+ to 16+ volts range. Different models, or sometimes even different units of a same model (production variations) may affect that, but typically most will be fine in that range.
So as long as your voltages are typically in that range, that is your primary concern. Ideal is 11-14 "normal" operations, and below 10 your unit will likely become erratic or not work at all.
Above that, your unit's power supply will have to "cut" power internally, generating heat which is never good in the long run, but this should typically only happen for short periods at high-rpm operations, or if something is wrong with wiring, as a battery will buffer the voltage at some point above 14.x (kind of like a spring, the more voltage "pushed" from the alternator, the more the battery will absorb, although it's also not good for the battery and most properly functioning alternator circuits are regulated to prevent too high of a voltage output.
Worst-case scenario, you plug your unit into the 24V voltage from a dual-battery trolling setup, at which point a large rush of current will flow, which is where the 5A (amp) fuse (or whatever your unit's fuse is) comes in, this large voltage difference (much higher than your unit can safely handle), causes the fuse to blow, protecting (hopefully) your unit.

Using the hose analogy above, voltage is pressure, and a lot of pressure on a small hose (the fuse, and your sonar), will cause the hose to burst, so instead of blowing the expensive unit, you blow the cheap fuse out first (hopefully).
Keep in mind, fuses are "worst-case" protection, not a guarantee, if your unit is designed to work on 12V at 1A, and you hit it with 24V at large amperages, at least 4A "excess" is going to reach the unit in the moment before the fuse blows. In fact it doesn't hurt to "fuse down" your unit (especially during installation) to a much smaller amperage (if it says "max current: 1A " put in a 2A fuse until done getting it all wired up).

Finally, get yourself a cheap digital circuit meter ($5 from Harbor Freight, probably $10 from WalMart), set it to "20V DC" setting, and test your battery voltage with the sonar turned on and voltage displayed on the screen. If you see "12.6" at the battery, but only 11.4 at the sonar, you know there is something causing a large drop in voltage, either insufficient wiring, or more likely, something "shared" on that circuit that is pulling power away from the sonar.
With normal "small" (16-18ga) wiring, you may see a 0.2V-0.4V drop from battery to unit if all connections are good and it's the only thing on the line. Much more than that and you need to start looking for what's going on.
IF you don't want to/can't buy a separate meter but can move the sonar power connections, move them straight to the battery (with the fuse ALWAYS installed properly though!!!!), and use that for a readout at the battery.
Hope that helps clarify some, but as long as you are seeing 11+ during operation, you definitely don't need to worry about it unless you just want to for some reason.

It seems to me these answers, while accurate, are far more complicated that what he is asking. The voltage readout on the fishfinder is to monitor the status of the battery it is connected to. If only the fishfinder is connected anything above 12 (and likely less) is fine. If it is connected to the same battery that starts the engine you want to be sure you save enough power to crank the engine at the end of the day. I would be getting concerned if the reading dropped down near 12. It can also tell you if the alternator on the motor is charging the battery while the engine is running. It should read 13 or higher then.

Just trying to cover the possible concerns he might have had that caused him to ask the question, it can easily drop below 12V if it's on the bow and has a thin wire run, or shares a power circuit with aerator pumps, other electronics, interior lighting circuit, etc, for instance, or in my boat, had some bad wiring (ground wire nearly completely out of the wiring harness causing it to act odd one day and fine the next).

Figure it doesn't hurt to be helpful and thorough, if he doesn't need that much detail, someone else might down the road that comes across this. Other simpler answers were above mine so didn't hurt to flesh it out some.

I agree Chris, detail is always good if someone is willing to listen. I've learned a lot that way. I, too, was trying to figure out just what he wanted to know, not critize your answer. I think we is asking "why is that read out there?" although I really don't know what he wanted to know.

That's good info from all above respondents. Thanks to all