OP here: We recently released a software package that lets a boat owner monitor their vessel without any proprietary hardware. It's open source (<a href="https://github.com/floathub/sfh" rel="nofollow">https://github.com/floathub/sfh</a>), and the above linked article is essentially a walkthrough of how to get it working on a Raspberry Pi. Thought it might be of interest here, please let us know if any questions.