A couple of updates for this thread from recent monitoring:
San Diego Lifeguard Services dispatch ("Lifeguard Dispatch" / "Station B") is located on Quivira Ct, west of Sea World at the mouth of Mission Bay. In addition to their headquarters, there is a main lifeguard station at every major beach (in a permanent building with glass tower windows, not the portable metal towers). There's one at Ocean Beach, one at both South Mission Beach and Mission Beach, and La Jolla Shores. I think there's one at PB too (but it might just be "Tower 23").
At least some of the Lifeguard Tactical channels actually are divided up geographically, with the guards in the small towers communicating both with each other and with the main lifeguard station covering that geographic area. Here's what I've observed:
Tac 1 - Ocean Beach (low tower numbers from 1 to 6 or so and station "OB").
Tac 2, 3 and 4 - Mission Bay boat patrols, as well as general car-to-car Tac traffic.
Tac 5 - South Mission Beach ("South Mission", tower numbers in the teens).
Tac 6 - Mission Beach & Pacific Beach ("Mission Beach", "Tower 23", and towers in the teens and twenties).
Tac 7 - La Jolla Shores (tower numbers in the low thirties).
Fortunately, the tower numbers start in OB and work their way north, so it isn't too difficult to figure out.
I haven't quite figured out the mobile unit IDs yet. It consists of one or two numbers and a letter (eg "2 Lincoln" or "24 Sam"). So far it seems very similar to the PD unit numbering (with L being the watch commander, possibly Lieutenant, and S being a supervisor or Sergeant), but I don't know the Lifeguard Services organizational structure to know for sure. Boats use the "Rescue" callsign, eg "Rescue 1."
That's all I've got for now! Will have to watch some old Beach Patrol San Diego reruns one of these days to fill in the rest.