Here's a very experimental bathysphere for exploring the depths... or at least, the depths reachable with 3d-printed parts. To make this work, you'll need to print the gasket with flexible filament, like Ninjaflex. Inset the gasket into the rim of the upper sphere, and tightly bolt them together with six M5 x 30mm bolts. Don't forget to use washers!

Attach a tether to the handle on the shell, fill the bathysphere with heavy ballast (or attach a weight to your tether line), and see how far down it'll go without letting in water.