I don't know about Xanterra or Aramark, but I do know that at Princess Cruises, your hours of service play into the 401(k) picture. Seasonal employees are not full-time, and therefore, not eligible to participate in the program, but the time in does help with vesting, or ownership, of the company contribution for those who switch to full-time employment at some point.
I used to do the benefits for Princess so if you have any other questions, please let me know.