Well, two suggestions are duct tape, and multilayered cardboard. Offcourse, you can 'make' your own multilayers by gluing them together. Also, perhaps caulk, or homemade crack sealers.
You will also want to use a good sealer or sealing paint on the outside.
I am not sure what you are building it for, but there are races with full size boats where people paddle them.
I found this link on such a site: (It has various tips to building a full size boat)
http://www.gcbr.com/tips.html