Outdoor recreation improves quality of life for individuals with tetraplegia, however providing safe opportunities to engage in these sports has many challenges. We describe the iterative design and field evaluation of Tetra-Ski, a novel power-assisted ski chair. Users control Tetra-Ski with a joystick or Sip-and-Puff controller either independently or collaboratively with a tethered skier through our novel shared-control scheme. We also developed a training simulator to help prepare users prior to their skiing experience. A field study with eight participants and interviews with three trainers who used Tetra-Ski showed that Tetra-Ski is usable, enjoyable, and that the experience has a positive psychosocial effect on users. Furthermore, the shared-control scheme developed for Tetra-Ski proved crucial for supporting the unique abilities of different users, suggesting that a shared-control approach could enable broader access to less-dependent forms of outdoor recreation in the future.