It is easy to find simple depth sounders that are interoperable and have open-ish interfaces, but you would be hard-pressed to find a device that gives the full water column and provides an open interface to do so.
This is a reference implementation to communicate with Vexilar SonarPhone fish finders (http://sonarphone.mobi/).
This project, and its author(s), are in no way affiliated with Vexilar. This was created for educational purposes only on the author's personal time, and is released without any warranty whatsoever.
The protocol was reverse engineered by monitoring the packets between the device and the manufacturer's app. Hex dumps of the captured packets were compared against what the app displayed and was commanding to the device. Through some trial and error, a portion of the protocol was decoded.
I've only built this on Linux, but it will probably work in Windows.
Requires Qt 4 or 5.
$ git clone https://github.com/scherererer/SonarPhony.git
$ cd SonarPhony
$ qmake-qt5 -r
$ makeTo run, put your device in the water to turn it on, or shove a bit of wet paper cloth across the metal contacts for an in-air test (I don't recommend you do this too much, my experience has said running these in air isn't a good thing but it is for the consumer so it's probably fairly hardy). From your computer, a wifi network should appear (in my case it is T-POD-37A) and enter your wifi password as the manufacturer instructs. Then, run the program and hit the "run" button. The screen should start filling up with data.
This software has only been tested with the T-POD because it was the cheapest one money could buy, however I suspect the protocol will be very similar on the other variants since it has to interop with the same app. If you are using one of the other products I would be really interested to see a full log.
One of the things I have not fully experimented with is how the "master" password protocol works, and so I strictly used the default password. If you're having trouble controlling the device or getting pings back and you changed the password, that may be why (although I haven't attempted that specific edge case yet).