There's a hybrid impatiens called Sunpatiens which were developed to withstand heat and full sun. They kind of remind me of a cross between New Guinea and regular impatiens. They are also unaffected by downy mildew.
Check this out -
http://sakataornamentals.com/_ccLib/attachments/topcrops/SunPatiensFloatingGarden.pdf
Check this out -
http://sakataornamentals.com/_ccLib/attachments/topcrops/SunPatiensFloatingGarden.pdf