waterskiing focal lenght?

was wondering if anyone could tell me how long the pull rope is. taking the kids out and might not get another chance, and wondering if my 70-200 might need a little help. on a crop, should I pickup a 1.4x?

Pulls ropes range anywhere from 40 to 60 feet if I recall correctly.

Your 70-200 will work fine.
In a boat bouncing over the wake, you'll need to frame loose so you can keep your subject in the image.

On 1.6X a 70-200 is going to be perfect. It will be a little loose, but you need that to keep the skiier in the frame and allow for straightening horizons.

You need to focus on the skiier as the rope draws tight and then never try to focus again (either flip the lens to MF or use back button focus). Once the boat is moving it is hard to accurately focus, and since the rope is a fixed length you only need to focus once per skiier.

For the focal length, here is a single shot from last summer. This is 300mm on a 1D Mark IV. I show the uncropped and final version so you can see how I shoot a little loose to deal with all the bumping.

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Just as another thought. Pull ropes for skiing can vary widely in length. Although I can't remember ever seeing one less than 50ft (that wasn't a specialty rope) or more than 100ft. We have a rope that does 50, 60, 75 and 100ft.

Also remember, that if they are very new to skiing, a longer rope will make it easier to learn to cross the wake (smaller, farther out) so the shortest option is not always best.

I do agree with focusing once, giving yourself a little leeway with DOF and also backing your zoom out just a little and then cropping the final version.