I encountered a strange creature (or plant?) in shallow, calm, beachy waters in the Florida keys. It looked like a balloon or bag, about 4 inches long and 3 inches in diameter, filled with pale brownish slime, and it seemed to be dug in or attached to the sand at one end. Does anyone know what this was? We saw several, some less plump and robust. Thanks!

I'm not finding a critter that matches your description, but am thinking it's a seaweed.

Did you happen to see any of it washed up on the shore?

Hi. What you are describing sounds very much like it might be a sea cucumber...

http://en.wikipedia.org/wiki/Sea_cucumber

There are many different varieties, and some do burrow into the ocean floor. Though they are considered nocturnal, and to the best of my knowledge, aren't generally found close in to shore.

It's hard to say for sure. Hope this is indeed what you saw. :)

Likely not a sea cucumber as, as Thomas indicated, they are not found close to shore. They live in the benthic layer: deep:
http://en.wikipedia.org/wiki/Benthic

It Ocean Slime and it's full of bacteria caused by the ocean being out of balance, caused by pollution, overfishing, warming waters and some combination of the three are to blame. Go to the web address below to find out more details:

Oceans on the precipice: Scripps scientist warns of mass extinctions and 'rise of slime' (http://www.eurekalert.org/pub_releases/2008-08/uoc--oot081308.php)