this is a pretty cool open hardware and open source echosunder project. Still in development but pretty interesting at least: https://github.com/Neumi/open_echo
copied from the Github:
An ongoing open-source software and hardware project about building a sonar for testing, boating, bathymetry and research. Current hardware can drive a variety of commercial and DIY transducers and read RAW and interpreted distance/depth data. A python software interface lets you connect to Open Echo boards and read data and change settings.