I've seen a lot of human powered stuff just recently in my search for
info on building my own paddleboat. You can get some ideas here;

http://www.waterbiking.org/
http://www.humanpoweredboats.com

The paddle ideas seems the cheapest, simplest and surest way of
implementing human power, all it takes if you want to do it on the
cheap is an old bycicle, maybe one or two extra chains, a couple of
different size sprockets for experimenting and something to make the
paddle out of (can be wood), if you wanted to get fancy you could try
using the gears off a ten speed.