You might check to make sure the battery is not moving. If running on rough waters is causing it to shut down, then there may be something hitting the post causing you to kick the power off.

Do a battery test. Borrow a good battery, one of your cars or friends, or you could buy another battery. After the test if it still does it then take the new one back. Hook the depth finder to the borrowed battery and run down the lake a few times. If it clears up then you need a new battery. If it doesn't, then there is a short in the wiring somewhere. Good luck.