I've never been to Skagway, but the pictures I've seen show a dirt dogsled ride. I just got on line and it looks like the M&M helicopter tour and dogsled ride, or Temsco helicopter tour are pretty much what we did in Juneau. If you've booked with one of these companies I think it would be the same experience. If not, you can contact the company you booked with and make sure it will be on snow and not dirt. What other ports are you going to?
Have a great cruise!