According to the NIH
(http://www.nlm.nih.gov/medlineplus/ency/article/003128.htm#Common%20Causes):

Common Causes

Dietary changes, diarrhea, and malabsorption can cause floating stools. Most
causes are benign and will resolve when the infection ends or the bacteria
in the GI tract become accustomed to the changes in your diet.

Patients with the following diseases can also have stool that floats:

cystic fibrosis
gluten-induced enteropathy (sprue, celiac disease)
idiopathic steatorrhea (fatty stools with no known cause)
Disaccharides deficiency (insufficient amounts of the sugar-digesting
enzymes lactase, sucrase, or isomaltase)
short bowel syndrome
biliary atresia
abetalipoproteinemia

Home Care

Floating stools alone are not diagnostic of anything so they do not require
home care. If a change in diet has caused problems, try to eliminate the
offending food(s).

* * *

http://yarchive.net/med/floating_shit.html

   Floating BMs are entirely due to small gas inclusions, a fact proven
long ago by intrepid researchers who centrifuged various samples to get
true solid densities.  While gas is increased by fiber, unfortunately
it's also caused by lots of other things.  For example, a fair fraction
of people make a lot of gut methane, and this tendency seems to be
inherited as an autosomal dominant.  Their stools float no matter what
they eat, virtuously vegetarian or not.

                                     Steve Harris, M.D.