Have seen this species in my neighbor's pool - they are tough to die compare to flies, etc which doesn't last long. They are very attracted to the water - they are rather large for sweat bees. They have a metallic green upper body while the lower bodies look strongly like a wasp even down to the shape and colors. Noticed in August.
Malus2006
pford1854
http://bugguide.net/node/view/74478
tried to match up several images with BG.