I don't think "floating" would work. I had some planted in pairs of my husbands dress socks...shhhhhh, he still doesn't know. And tucked into rocks in my stream. They were amazing! When I get home I'll try to remember to post a couple pics.
I finally found where I read aboiut floating impatiens in the pond. It is the ivillage GardenWeb forum.
The poster said she/he put impatiens with no dirt in a little plastic pot and put the pot in a can cozy (like for beer cans). Said it works well. I think I'll try it this year.
I read more on the topic of floating islands for pond plants in the same forum. One person made an island with two washable furnace filters, Good Stuff, etc. They planted it with a flat of impatiens. Photos are really impressive. I'm starting to get that old "let's go to Lowe's" itch!