I use water kiting as a cross trainer for skiing as far as leg strength, balance, and a bit of core goes.
I boulder for overall strength, especially core, try and get 1-2 hours in before hitting the gym upstairs for free weights.  squats, lunges, pilates, leg press, and the usual upper body stuff- just because.  Box jumps are great for power/explosiveness.  I swim for endurance training, its great to swim the day after a free weights session, it gets everything moving and keeps your body looser.  I try and do about 5, 10 minutes with a jump rope daily- the mild impact is great for both muscle/bone and not nearly as hard on your body as running.If I feel like i need more work on my legs as far as endurance goes I will spend some time on a stair-master.
I'll try a little of everything-  I tend to push a little too hard with any winter sports- having something to fall back on to make up for bad technique or sketchy judgment can't hurt  
