We doubt you could really live in [Pena’s] PVC submarine, but now the song’s stuck in our head anyway. Although the post is in Portuguese, you can get a pretty good idea of how it works, and translation software is better than ever. Transcending the language barrier, there are videos of just about every step of the construction. We didn’t, however, find a video of the vehicle in the water.
The plumber’s delight has modified motors for thrusters, and a camera as well. Epoxy potting keeps things waterproof. We’ve seen candle wax used for the same purpose in other builds.
Our Portuguese may be wanting, but we couldn’t figure out how the sub worked with ballast or any other scheme to change its buoyancy dynamically. There was talk of using PVC tubes that hold air and trimming them to get neutral buoyancy, but there seemed to be no way to change it on the fly, so to speak. Perhaps someone with better language skills can help us out in the comments. We did note that the tether cables had an interesting buoyancy measure: pool floats cut open to make floating conduits. Not a bad idea.
We’ve seen PVC subs before and they can be surprisingly inexpensive. We’ve also used baking powder for buoyancy.