to make sure the problem comes from the depth sounder, you should disconnect the negative wire of the supply. If you mean the B&G Hydra 3000 depth sounder, this would sound strange as it is fed with 12V coming from the power supply located under the nav station.
However, disconnect it and check your fault device.
If your depth sounder is a FURUNO one (for instance) then it is working with 24V and may create a leak on the negative 24V.
If the depth sounder is not involved, then you will start a long process of checking every item working with 24V. You will need to disconnect the negative wire on each one and do the test every time. You should start with the outside items (furlers, nav lights...), then check every item in touch with water, then every item that includes a motor.
if you recently added a new equipment, check this also.
