The reason that 50mm are so cheap is that they are mass produced and became cheaper to build. 50mm is about equivalent to the human eye. So what your eye sees, the camera will capture. The 50mm f/1.8 is perfectly fine for the D300. You will see that it looks like it was built cheaply (and it was) but the optics are impressive.
I have the Nikkor 18-200 also. It's not bad but indoors it is horrible. So my nifty fifty is on my camera most of the time.
As for the Tamron... it's not getting great reviews. Sounds like the auto focus is slow. So in an constantly moving sport like wakeboarding, you are more likely going to get out of focus pictures.
But check the reviews yourself. http://www.fredmiranda.com/reviews/
You want to improve your photography dramatically? Get the 50mm f/1.8 and a SB800 flash. A good flash will make more of a difference they any lens.