I have a 70-210 D and love it. I bought it on ebay for around $200. You can get the non-D for around $100, it is the bargain. Non-D is really the bargain lense to get. A little slower focus speed on the non-D but not that much. For wake board pics it won't matter. The subject is always the same distance - at the end of the rope! I think these are better than the Nikon 70-300 and the 55-200. Not sure about the Tamron. They are generally not as highly touted as the nikons. Check out Ken Rockwells reviews here. He is very controversial, but I am sure he knows more than me. If you have a lot of money get an 80-200 f2.8. I need gas money.
http://www.kenrockwell.com/nikon/nikkor.htm#af Here is a pic I took yesterday with my D50 and 70-210D. Only cropped not shopped. I can not upload a full quality or size image but you get the idea.