I run long shallow ones on my superjet. Much improved handling. The boat still slides nicely when it is flat, but it keeps the back end from kicking out while I am turning hard. Mine are nothing like the ones in this thread. More like the TBM race style. I know from waveboarding that a long shallow fin gives just as much hook up as a short deep fin.
Speaking wakeboarding, Short deep fins reenter the water aggressively and kick you hard when you are doing powerslides, butter slides and rotations. Anytime you break the fin out, they reenter very rough.
Long shallow fins don't hit the water as soon or with as much leverage. But, they give you just as much traction because the same amount of fin is in the water, Actually, more traction, because they have more leverage to keep the board straight. You loose traction sooner, but that is good, because you are trying to spin or slide.
I found on my superjet that my sponsons help a lot. They reduce the sway in the back and grealy improve stability. Like how front sponsons eliminate roll, rear sponsons eliminate sway. However, like front sponsons, you can still make it roll with a little more effort. The back end of the ski still swing around, it's no different than breaking the fin of the wakeboard free. It take more effort to do the trick, but you have much more controlled set-up
Mine measure approximately 22" long and are about 1.25" below the bond rail. They have a long sweep to them. The tail of the sponson that I have doesn't matter that much, every curve does something a little different and wont translate directly from wakeboarding. If you are looking for something pre-made, TBM has a good set.
I haven't had time to try out too many fins, and we don't have the killer surf they have in Austrailia, Cali or even the US Easy coast to test, but so far I have been pleased.
I messed up and made mine 2 short. It was my intention to use the stock bond rail holes to hold them on, but I was working fast and failed to account for the tail of the fin. I should have made the upper mounting portion (or the whole fin) 1" longer so it would have reached that forward mounting position. Make sure if you have turf, you run fender washers.
I feel like deeper fins would let me carve harder, but would prevent me from sliding and spining more.