I also designed a bunch of submarines and wanted to build one for the masses. Until my friends told me I was crazy and I would kill myself. My final design was

I also designed a bunch of submarines and wanted to build one for the masses. Until my friends told me I was crazy and I would kill myself. My final design was using scuba tanks to equalize the internal pressure of the sub with ambient pressure at specific depth. This was a clever design but limited it's usefulness to 30f as well. It simplified the construction of the sub but limited its use as it required depressurization following the same rules applied to SCUBA divers. So at 30ft you could be submerged for a while, but any deeper it limited the time you could remained submerged unless you depressurized before surfacing.