Testing for boating electrical issues

Just finished working on my front Humminbird sonar not powering up. I put this unit into service last year after running new wire from my battery to the unit on the console and continuing on to the bow. I took great pains in soldering all connections, crimping and taping this solve a problem I had with the console unit blacking out. Everything has been fine until a couple of weeks ago. My bow unit would not power on. Check everything had voltage all the way to the unit head. Was making prep to send unit back to the factory. I decided to try one more thing. although I had voltage to the head I had no current (amps) I put a test light on it and it would not light it up. The fuse holder was bad, it would allow voltage to be read thru it,but it would not pass current. I guess the point of this posting is to not rely on voltage testing to ensure power to anything on your boat. Make sure all your connections will pass current.