900 return? !!!!! What airline - solid gold transit? I have to fly between Van. Island and either Calgary or Edmonton several times a year. Seat sales always get me about $130 each way. Even with the taxes and gas surcharges, it's still always about $300 round trip from Nanaimo airport.
As for lakes, depends where you are. Minnewanka is always good from about mid-July onwards. Vis gets better right up until freeze-over sometime in December. Vis after spring thaw is always horrible (1" or so for May, June, sometimes July).
If you are near Edmonton, there's lots of nice lakes in Jasper. Funny thing - Jasper is about the same drive time from Edmonton as Minnewanka.
Jasper has Patricia, where the ice barge is located. Very cool dive. Minnewanka has a submerged hydro dam, bridge pilings and several other artifacts.
If you are in S. AB then Waterton is a great dive. There's not much left of the Gertrude, a side-wheeler that ran the lake for years. Mostly just some timbers now.
-S