Different woods, different uses
One of the positive things about teak is that it is practically unaffected by water. Most woods (particularly soft woods, such as pine) have cells that tend to retain their ability to swell with water (cellular osmosis), and to actually transport the water through the wood, as if the tree were still alive. Furthermore, since the tree has been cut, and is no longer in one piece, it tends to twist and warp, since it no longer has the torque of the rest of the tree to keep it in place. One thing that is done to counter-act twisting and warping is to use a quarter sawn plank of wood (it's length emanates directly from the core of the tree), this tends to give a piece of wood that is more stable and will grow and shrink, rather than twist and warp. Plywood is also more stable as it has different grains in different directions all laminated together, but the laminate should be as water resistant as possible.
Teak however is so naturally saturated with it's own oils, that water basically beads up on the surface, kind of like how a duck never gets wet. As far as this property goes, teak is unquestionably the best. Other types of wood have similar properties in varying degrees. Once again however, aluminum would be even better.
It just so happens to be that I found this thread because I'm working on exactly the same project as you. I have a Venture 21 that I'm totally redoing, I got it on the side of the road with a big free sign on it. Needless to say she's needed a ot of work. I happen to be a metal worker as well, and was actually going to do something similar, make my own kick back rudder. I was just going to buy the blade on e-bay and manufacture the rest. And it just so happens that I found a set of two blades on ebay item #260561129457, they are 31'' long, 9.5'' wide, and .750'' thick. They are fiberglass with alots of stainless reinforcement rods in them. Thus my own question? - Is it big enough? the auction ends in 1 hour, so I hope you see this and reply in time. If your interested I will sell you one at half price of the auction.
Thanks and good luck with your rudder - Gorchew