Welcome to Jon's Mini-Sub Gallery...

Here's a deep-water sub, based somewhat on the Deep Rover submersible (designed by Graham Hawkes, the guy who did Deep Flight). It has a five foot diameter, four inch thick acrylic sphere as the pressure hull. It would probably be good to about 2400 feet, although it wouldn't be very fast. The visibility would be unbelievable, though...

BTW, there's an error on the Deep Rover link page above, it can actually go to 3000 feet, not 300...

This is another idea I had for a two-person sub with pretty good visibility and deeper capability than my main design. It uses half of an acrylic sphere for the front viewport, and an acrylic tube for part of the pressure hull. The diameter is four feet, and the hull thickness is two inches.

For an example of a commercial sub along these ideas (but much larger), see the tourist submarine SeaView being built by US Submarines.