Castable Depth / Fish Finders

I can't speak to the fish part but from the nerd perspective, the range for bluetooth class 2 is only 10 meters and class 1 is 100 meters. One of them lists a bt range of 150 feet which is really odd and neither of the ones I looked at on your link specifically mentioned the bt class. Regardless, you're limited to what your phone is capable of doing. Most phones are class 2, some newer ones are class 1. Point being, you may not be able to drag the thing more than 10 meters behind you...not sure if that's an issue but I guess the whole point of a remote fish finder is having it remote.

Where would you be using it? I think it would be a bit cumbersome in your new kayak while attached to a fishing rod. Also I try to keep my phone in a dry bag most of the time as you never know when you might take a swim. If you want a fish/depth finder the ones you can put on detatchible ram mounts with either a fixed transducer or a suction cup style work well. Lowrance and hummingbird units can be found pretty cheap this time of year. We used to have an old one that we stuck to a Jon boat that worked well. There is also one that screws on sort of like a trolling motor and just hangs off the side of the boat.

With any of the lower end units you will get depth and a graph of the bottom structure. As for the fish, it takes some getting use to. The fish come up on the screen as little frowns. An active school looks like a child scribbled all over the screen. It's often hard to distinguish weather you are looking at floating debris or a fish. The more money you spend the better clarity you have. Rarely do I see a fish on the screen then catch it. I often just look for good bottom structure and signs of active fish on the graph.