This is a fun vessel, maybe mostly because of its very interesting creator, Brian Walker aka the Rocket Guy. Brian is an inventor and in fact a successful toy inventor who has dabbled in a number of areas that should be of interest to Sub-Log readers including, most recently, rockets!
The HydroSub is a craft that is one part submersible and two parts boat, something like the AquaSub [1], which can submerge the crew compartment while keeping its occupants dry. A large part of the boat stays on the surface which makes it handy for supplying breathing air and depth control.
Check it out and read about Brian's other great projects:
