Most, if not all, fish finders have a low input voltage cutoff that will shut off the unit when the voltage is too low for NORMAL operation. My Lowrance 527 shuts off at 10.2 volts. If I recall, all of my Humminbird FF cut off at 10.3. They all work perfectly up until they reach the low voltage cutoff. I doubt it is a problem with power supply voltage. More likely is signal degradation caused by a bad connector or conductor. The first step is to check the transducer in the water. This will tell you several things:
1. If you get a good signal with the transducer in the water you know it was the mounting material/method and that the transducer, interconnecting cable and head unit are good.
2. If you still aren't getting any return on the display you will know that it wasn't the Goop.
If you determine that the fault does not lie with the mounting, then you need to check a few more things. See if you can connect another transducer to the head unit. If the other transducer works, then your transducer is bad and needs to be replaced. If you still don't get a signal then the problem is with the head unit or connectors. The most likely fault is corrosion on the connectors or a frayed/broken conductor. Corrosion on the pins of your transducer plug could cause diminished signal strength from the transducer, thus causing the depth reading to grow worse over time. Check the connectors on both the plug and head unit. The connectors on Humminbird units aren't really very watertight. Even a small amount of corrosion might cause some signal loss.
You can clean the connector pins with a weak acid solution. Use an acid brush to apply and gently clean the connectors. You must take care so as not to bend or break the pins. Use some electrical contact cleaner after to get rid of any residue. It is a very good idea to use a corrosion inhibitor on all electrical connectors on you kayak. In the past, I used a thin coating of dielectric grease on connectors. This will inhibit corrosion and also helps keep water out of the connectors. The downside is that it holds sand and dirt particles and if you use too much of the grease it can degrade the signal. Use sparingly and you'll be OK. I have since switched to Fluid Film for protecting my electrical connectors.
http://www.fluid-film.com/
You can get Fluid Film from Hook1 or the John Deere service center off I8 in El Cajon. Hook1 only has the cans of liquid Fluid Film. The John Deere place has it in aerosol cans.
You can get dielectric grease at West Marine or at many electronics and battery shops.