The simplest solution would be the depth finder of your choice, then pick up one of these 12 Volt DC 7mah batteries.
Lithonia Lighting 12-Volt 7mAH Replacement Battery-ELB 1270A R3 at The Home Depot
And one of these float chargers to re-charge the battery after a day on the water.
Automatic Battery Charger - 12V
This way you don't have to lug around a heavy car battery. Just place the small battery in a plastic container like Tupperware, punch a hole in the side and run your fused positive and negative wires from the depth finder to the terminals on the battery.