any sprot where you are moving and controling your balance in a very coordinated way will contribute. even thought they apear the simallar i don't think that skateboarding will directly improve your wakeboarding skills. the balance is very different because you are moving freely instead of being pulled, also the turning is very different. i dont' skatebard anymore, i inline skate and i think that balance helps just as much as skateboarding would.
the main point here is, nothing but wakeboarding will really help you get really good at wakeboarding. however lots of other things help- trampoline, skateboarding, skating, snowboarding,i would imagine even a pogo stick to some extent.