Please don't consider this official, but it's what I know based on historical schedules.
The only one I know of that is year round is with Renaissance Adventure Guides. They do locations in Golden and I think Englewood or Lakewood or some arcane locale. Check their website.
Others tend to start around or after the New Year:
Boulder - CU Rec center if you have access (i.e. student, faculty, etc.) (not sure when or who I have to bribe to let me go to these)
Arvada - Meyer's Pool through CWWA (Sundays)
Denver - Confluence does sessions at DU (Tuesdays historically)
Broomfield - They have been doing pool sessions on Saturdays at old rec center. Low attendance, awkward entrance.
Longmont - I think they tend to have something but I don't know when or where.
Preferences? Meyers pool is nice and big and relatively convenient if you're around on a Sunday. DU is nice because it is late Tuesday and what else am I going to be doing then? It's obnoxiously far if you're coming from Boulder though.