You can build a glider that works underwater, much like a sailplane works in the air. Really. It'll fly like a glider too, with gravity pulling it along as it descends. Sure there are differences between an air and a water glider. The underwater version will have smaller wings than you might expect because of how dense water is compared to air. And it's probably not going to move that fast. Descending, the glider has the force of gravity pulling it down and the wings help translate that downward pull into forward motion just like a glider in the air. The clever trick that underwater gliders perform is that at the bottom of its flight, at the deepest point, the craft needs to increase its buoyancy and have its wings start operating in reverse, producing a lift force that pulls down, resisting the craft's new buoyancy and translating it into forward motion.
These gliders can operate on a small amount of energy, slowly descending and ascending, and for this reason you see a number of autonomous underwater gliders being used to collect ocean data. The WHOI glider works by varying it's buoyancy by one liter which is just 2.2lbs. That's not a lot of force and it also wouldn't take too much electrical power to pump one liter in and out of a buoyancy chamber. Kind of nice for staying onsite for weeks or more and taking samples and readings from a range of depths over a period of time. See the links below for more reading.
In terms of suitability for a people-carrying, homebuilt type project, well I don't know. One apparently very successful example is Karl Stanley's CBUG which he's used for years as a tourist sub. See the photo and link below and check out Mr. Stanley's website for more photos and information. Another great looking submersible is the original DeepFlight craft which is not however a glider and has electric motor propulsion. I've included here mostly because it has wings and it looks good.
Autonomous research gliders:
Wired Magazine Navy glider article... [2]
Here's Karl Stanley's CBUG, a homebuilt gliding submersible that is good to 500ft and it's for sale!
And finally here's an interesting, probably more familiar, powered submersible with wings. These vessels weren't built as gliders but they have the best speed of most any submersibles I know of and they just look like they'd be a blast to dive. I don't know how this translates into doing useful work but I guess that's up to the DeepFlight founder, Graham Hawkes to figure out.
DeepFlight article in San Francisco Chronicle... [4]