There have been previous threads on this subject, and lots of concern over the safety of such a device. The one I've made uses easily available, and easily worked parts, and uses them at pressures and circumstances they are manufactured for - ergo the risk of an injurious failure is minimal.

Mine is made from PVC pipe, incorporates a pressure gauge, and uses residential water pressure (55 psi in my area). This allows me to test stuff down to 120'. Good enough for my purposes. Per the irrigation supply I bought the parts from, my pot should handle well above 100 psi.

First some pictures:
IMG_1617.jpgIMG_1618.jpgIMG_1619.jpg

Parts list (4" diameter, good for a GoPro housing, and lots of lights):
  • 12" of 4" diameter PVC irrigation pipe (flared) - in the future I would use regular Sch 40 pipe and a F/F connector. The flare doesn't fit tight and I have minor leaks there.
  • 4" threaded end cap (female)
  • 4" threaded (male)/slip (female) connector
  • 4" to 2" reducer (s/s)
  • 2" slip to 3/4" threaded (female) reducer
  • 3/4" m/m threaded insert
  • 3/4" threaded/threaded/threaded "t"
  • 3/4" threaded to hose connector
  • standard spigot "y"
  • 3/4" to ?? reducer and matching pressure gauge
  • PVC glues
Use teflon plumbers tape and glue to assemble and let it sit for 24 hours.

To use the pot: Insert the test object via the threaded cap and then fill with the water hose. Use the "Y" to vent any trapped air. Open/close the water valve slightly to adjust the pot pressure. If your pot is completely sealed (probably not at the threaded cap) you can then turn the water off. If not then leave the water turned on to maintain the pressure. Let your device 'dive' as long as you feel necessary and then turn off the water and slowly vent the pressure via the "Y". Remove the threaded cap (you may need a strap wrench or other device) and check your test object.

I've tested my GoPro housing, a homemade flat lens for it, some flashlights and strobes.

Safety considerations: since the pressuring agent is water (non-compressable) and therefore non-expandable the explosion hazard is minimal. The pot could crack, but is unlikely to explode. The fittings, if improperly glued, could separate and act as a short distance missile. Aim the pot's ends appropriately.

Finally, using the same plans a larger (but far more expensive) pot can be made.