Homemade submarine plans, or homebuilt submarines plans, for actual functioning submarines that carry people are a little hard to come by. Here's what I've found:
George Kittrege K-250 and K-350 are one-atmosphere dry submarines and are the only such homebuilt submarine plans that I've found. This design seems pretty popular with homebuilders.
The SportSub from International VentureCraft is offered in kit and fully-built form. This company has a series of ambient pressure, semi-dry subs. AutoHover, sonar and other features are available and these craft seem to be used in the resort trade.
Delta Wing Submarine is a semi-dry single person craft that is designed for divers. First seen in 1968 in Popular Mechanics, the sub is slightly buoyant and uses the two propulsion motors to descend and ascend. Not much information here on performance but you're supposed to be able to exit the boat in less than 5 seconds!
www.psubs.org/store [4] PSUBS has plans for 28.50 USD
Silent Runner II is a semi-dry sub and a diver transport vehicle that is not bad looking. Built around an aircraft drop tank, this model features two trolling motors, ballast adjustment and I'd say, some retro cachet.
www.psubs.org/store [5] PSUBS has plans for 28.50 USD
AquaSub from Robert Riley Enterprises has plans for what could be described as a dry pontoon boat/sub that allows the pilot compartment to descend to a depth of... three feet. This one's been around since the mid-1970s and maybe you've seen it in Popular Mechanix or Popular Science.
www.rqriley.com/aquasub.html [6] AquaSub plans available for 45 USD and the estimated cost to build is 1500 USD.
And finally, just for good web browsing, here's a page listing images of 45 historic submarine plans: Submarine plans [7]