Hi again,
First to thank those who responded to my question and second to followup and let others know who may have been wondering the same thing I did yesterday and want to look for the Yellow-billed Loon...
Access to South Platte Reservoir is much easier than I expected or assumed. As many of us know, sometimes reservoirs in CO are very tricky to view birds on - restricted access, distant one-side-only viewing from very far away (I'm looking at you, Lower Latham!) with sometime terrible light angles, etc. South Platte is indeed large, but has two very user-friendly viewing spots on the south side, one on the SW corner, another on the SE. Although the loon is likely to be quite distant, it is generally pretty easy to pick out when it is at the surface. (When it's diving, sorry, we can't help you there.) Bins are fine for seeing that the bird is present, but if you want to see better details that distinguish it from Common Loon, a scope is recommended. If you go today, more than likely someone at some point will be there with a scope if you don't have one and will be kind enough (I would hope!) to give you a look through it if you ask nicely with puppy-dog eyes.
Joe Roller sent me these instructions for getting to the viewing areas (although I corrected his spelling mistakes ;) ):
"Head WEST On C470 from S Sante Fe Drive == Hwy 85.
Take the next exit on S Platte Canyon Road., heading north.
Turn Sharp right onto S Platte Canyon Access road and follow road under dam to first lot, at
SW corner of dam or continue to larger parking lot and walk up on dam, look north.
Look for the birders.
NOTE the SW bay is in Jeffco, most of the Res is in Arapahoe Co."
Good luck!