This is a great use for one of those spare 'accessory' switch holes FW thoughtfully put in the dash.
You can simply hook up a 'bypass' switch so that you can turn on the depth-finder independantly of the ignition switch. Also, you could use a spring-loaded type of switch, so it would only be momentary contact - that way, you would only read the depth-finder when the switch is depressed.
The square hole is a standard holesize for several different switches. I buy many of my switches from
http://www.mouser.com Let me know if you need help in spec'ing out the right switch.
This arrangement requires a single-pole/double-throw switch. You also want one that does not have a 'center-off'. A center-off switch would have three positions, (UP, CENTER, DN), while you only want two positions (UP, DN).
In the 'normal' position, the depth finder is connected to the ignition switch normally, and when the ignition is off, the depth finder is off, so this position could be labelled "normal/Off".
In the other position, whether momentary or not, the depth finder only will be on.
The reason for the double-throw is that you don't want the switch turning on other devices in the ignition circuit when you read the depth-finder. In other words, you are switching the depth finder between its normal ignition circuit and a dedicated power source.
This is just one possible solution.