I too found Lake Okanagan to be
much warmer than the waters around Vancouver (except maybe the South arm of the Fraser River, in slow-moving sections off of Delta and the Southern part of Richmond).
When I lived in Kelowna, my swimming season would begin in early June (tried late May, and CCCCCOOOLLLLDD!!!) and often persist until the last week of September. I swam pretty much every day, several times a day. As others have noted, Gyro Beach is an especially pleasant spot for swimming in the summer. City Beach was a little colder. Okanagan Centre has a nice beach, but here too it was rather chilly. I used to swim in nearby Kalamalka Lake about as much (near Oyama, and also, Vernon). I found it was usually a few degrees warmer, which makes sense given that the depth and surface area is less than Okanagan Lake. Plus, Ogopogo enjoyed the cooler waters of the latter lake.
Harrison Lake is extremely beautiful with a very nice beach, but I came close to hypothermia trying to swim across a section. Nice along the shore, then you get out, and wholly fuck it is extremely cold. There is a sectioned-off part for sissies (foreground below) that heats up a bit more.
tripadvisor
Probably the best ocean swimming in BC (perhaps the country) is to be had along the east coast of Vancouver Island (around Parksville and Qualicum Beach, and also farther north, e.g., Miracle Beach near Comox)