you need to find out the density of the filaments you're looking at.
For starters on the tpu: ninjaflex and filaflex are the most common.

As long as a 3d printed object is watertight - it will almost always float.
By it's nature most 3d printed objects are a thin solid skin and a mesh or cell like interior which is mostly air anyway.
So forget the density bit. As long as you don't print it solid (which would be daft) floating shouldn't be an issue :-)