I just got a BB from Cabelas and ran a few test on my bench. You have the choice of powering the unit with a 9V battery or the boats 12V supply. When using the 9V batt the actual output voltage was 11% less than the displayed voltage on the BB, so if you set the output to .650V your only really getting .578V, could be the difference between a fish or not!
Using the external supply @ 13.7V- 11.5V the output accuracy was excellent, no more than a .003V difference, not bad for a $99 piece of equipment. Now the real test....out on the water!