Find out where the power source comes from and check the following -
* Replace in line fuses as with salt water fishing they will carrode and lodge their way in the holder and make it hard to get out
* If they are wired to the battery then clean the terminals and the connection.
* Clean out the connections on the unit and the connectors themselves that plug into the unit
* Put a new battery in place if possible and test if it works as most sounders require a certain amount of volts to work or they will not switch on
You can take it the BLA and get them to look at it but I am not sure on cost etc when you take this path sorry
Cheers
Lee