Lowrance HDS 10 flickers off and on
If you only have room for one battery, it needs to be the largest you can fit, with the highest CCA you can find. The higher the Cold Crank Amps, the lower the impedance of the battery, and the less it will sag when a starter/pump load is placed on it.
If you have room, adding a second parallel battery will also likely cure the problem. In extreme cases, a diode isolator can be added between the batteries; this will absolutely cure the issue, and allow you to use a small, cheap 12V gel battery as the second battery for sonar only.