Here's a impressive, and exciting submarine project.  Well, it's exciting if you're interested in building pressure vessels with concrete.  Now that's an obscure branch of nerd...

Back in 1996, Wilfried Ellmer built, tested and used a 20 ton concrete submarine on an Alpine lake.  Residents of Lake Atter in Austria witnessed the launch of a pristine, streamlined, white vessel.  The photo at right shows the boat at launching before the fitting out of sail, rudders and propellers.

Wilfried states that the 18cm  (7 inches) thick hull gives an operational depth of 300m (~1000ft) and an estimated crush depth of about 900m.  He used portland cement reinforced with 4mm and 6mm steel in a 10 cm mesh.

This is a large personal submarine, weighing in a 14 tons before ballasting.  Check out the interior photo on the website which shows a roomy cabin.  Unfortunately missing are any photos of the sub actually diving or even maneuvering on the surface under its own power. 

Concrete Sub page

Here's a page describing Georgia Tech's entrant to the Eighth Annual AUVSI and ONR Autonomous Underwater Vehicle Competition,   This model actually took 12th place in the competition, which ran Aug. 3-7, 2005, with the University of Florida's Subjugator taking first place.

The Georgia Tech students describe the design and construction of their pretty functional-looking AUV.  There are photos and schematics and in the end you actually get to see the craft in the water.

  http://www.linuxdevices.com/articles/AT7101484471.html

For some great reading and creative ideas on homebuilt submarines, go to www.submarineboat.com for Doug Jackson's excellent documentation of his submarine project.  The name of the site hints at the design philosophy: this is a boat that submerges.  It actually looks like a landing craft but is a planing aluminum sub with an ambient pressure diver compartment...

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...

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 link below and check out Mr. Stanley's photos and track record as a tourist sub builder and operator...

The David Taylor Model Basin is one of the largest ship model basins — test facilities for the development of ship design — in the world.

David Watson Taylor designed and supervised construction in 1896 of the Washington Navy Yard's Experimental Model Basin (EMB). The Basin named for him was built in 1939 in Bethesda, Maryland, and contains a shallow water basin, a deep water basin, and a high-speed basin.

USS Albacore (AGSS-569), a unique research submarine, was the third ship of the United States Navy to be named for the albacore, a small tuna found in temperate seas throughout the world... Veteran submariners visualized a new type of submarine in which surface performance characteristics would be completely subordinated to high submerged speed and agility...

A rebreather is a type of breathing equipment that provides a breathing gas containing oxygen and recycles exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather a lightweight and compact breathing set for long durations in environments where humans cannot safely breathe from the atmosphere.

Rebreather technology is used in many environments:

• underwater - where it is sometimes known as "closed circuit scuba" as opposed to Aqua-Lung-type equipment, which is known as "open circuit scuba".
• mine rescue and in industry - where poisonous gases may be present or oxygen may be absent.
• space suits - the outer space is a vacuum where there is no oxygen to support life.
• Hospital anaesthesia breathing systems - to supply controlled proportions of gases to patients without altering the atmosphere the staff breathe.
• submarines and hyperbaric oxygen therapy chambers - where the gas in the habitat must remain safe. Here the rebreather is big and is connected to the air in the habitat.

As a person breathes, the body consumes oxygen and makes carbon dioxide. A person with an open-circuit breathing set typically only uses about a quarter of the oxygen in the air that is breathed in. The rest is breathed out along with the nitrogen...

Side scan sonar (also sometimes called side-scan sonar, sidescan sonar, side looking sonar and side-looking sonar) is a category of sonar system that is used to efficiently create an image of large areas of the sea floor. This technique is used for a wide variety of purposes, including creation ofnautical charts and detection and identification of underwater objects and bathymetric features. It may be used to conduct surveys for maritime archaeology ; in conjunction with seafloor samples it is able to provide an understanding of the surface geology of the seabed. Side scan sonar imagery is also a commonly used tool to detect debris items and other obstructions on the seafloor that may be hazardous to shipping or to seafloor installations by the oil and gas industry. In addition, the status of pipelines and cables on the seafloor can be investigated using side scan sonar. Side scan data is typically acquired along with bathymetric soundings and sub-bottom data which provides a glimpse of the shallow structure of the seabed. Side scan sonar is also used for fisheries research, dredging operations and environmental studies. It also has military applications including mine detection.