If I don't have a boat escort I'll typically swim along the shore from the swimming area, past the roped off area, to the spillway tower and back. That's about 2 miles round trip. I've never had any problems with boat traffic or park rangers doing this. It's pretty shallow along that path and the shore is never far off....
I would love to get an open water swim in myself and would feel much safer in a group. I hate to invite myself but would love to tag along if scheduling works out.
For me, it would have to be:
* in the evening on a week day (except Monday - I'm busy all day on Monday)
* in the afternoon/evening on a Saturday
* just about anytime on Sunday