I know nothing about Wastwater specifcally other than that I was there at the end of May this year. Clear still day and it was mirror calm (eventually I will post my blog with the pics) and looked beautiful.
I also know pretty much nowt about diving, but I know soneone who knows a bit about swimming (friend of a friend, former US Olympian swimmer) and I gleaned that in some lakes you can get 'cold water sink holes' which, if you're unlucky/stupid can effectively drag you down to your doom!
I dont know if Wastwater has these. You might be tempted to assume not since it's on the Nat Parks site (link gven in earlier post in this thread) as swimmsble. If it were me I'd check (pro) local knowledge first!