Homebuilt sidescan sonar is a pretty obscure hobby but a few people have done some good engineering and construction and gotten some nice underwater images. OK, actually the engineering and imaging does vary on the projects listed here but just getting anything recognizable as a shipwreck is an accomplishment in my book.
Dan Fountain has built several low-tech towfish and includes some sonar images he captured in Lake Superior. He's used fishfinder transducers and this seems like a good approach for homebuilders.
Some good design information here on side scan. Describes the theory of arranging transducers into arrays and provides some results from simulations of 4 and 6 element fishfinder arrays. Here's the secret: you can get better detail by NOT spacing the elements evenly. Read the article to get all the details.
Another website out there is posted by Jussi Kaasinen, a Finnish homebuilder. He hasn't posted an update since 2003 but he has some interesting images from his experiments.
And here's the king of amateur sonar (as far as I know), Sture Hultqvist of Sweden. He has built some high quality systems and recorded a number of very nice shipwreck images including the Russian subs S7 and S8 which are found in the '98 and '99 photo sections on his site.
Sture is doing sidescan work as a business and has built a number of towfish. The images below are used with his permission.
Sture's website: www.sonar.se [4]
and his old website... [5] which is no longer updated.
Towfish copyright Sture Hultqvist
Sidescan image of sunken S7 submarine
copyright Sture Hultqvist