Having dabbled in small boat building a bit, there are three things I have learned that may help you out...
1. It's easy to design a boat that will float and function, lots of simple / crazy designs work, if not work well. Heck, I rowed a log down a shallow river before. Poor design, but memorable
If your main goal is to have fun building it, putter around a bit and have people go "hey, look at that!" It's not to hard, just consider the worst case scenarios and make sure you are not putting yourself or others in danger.
2. It's very hard to design a boat that works well
. Even slight differences in angles can drastically change the performance characteristics of a boat. This is why good designers make good boats. If you want to spend a lot of time and money and have a boat that sails well, look into building from plans. (I don't think this is your goal, just say'in.)
3. Some times the "better" boat (the one you enjoy more) is the odd, cheap, poor performing but interesting idea boat that you nail together and play with for a year before it completely falls apart.
Just be sure to think through the bad "what ifs" first. What if it turtles, can anyone get stuck underneath? Is the water warm enough to be in it for a while if there is a problem? can you swim to shore if needed? Can someone else be along with another boat the first few times out for safety, until you figure out what the boat will do?
Sounds like a fun project, if you go through with it, post some pics!