Do I Need My Phone?
Yes, you do still need your phone, even if you have a Series 2 or 3 Watch. Right now all of the processing of GPS is done on the phone and the Watch mainly acts as a remote control / HUD for your stats.
Apple says it designed the GPS in a Series 2 watch to "last through a marathon." Unfortunately the 4hr average time of a marathon is a bit on the low side for all-day skiing. The Series 3 is rated about the same: 5hr on GPS and 4hr on GPS + LTE.
If I can deliver all-day battery life on the Watch and have it do the GPS work without the phone, I'll work towards letting you use just your Watch. Given how much the heart rate monitor kills the battery life, though, I'm a bit skeptical this'll be anytime soon.