While on a vacation with my 3D printers in tow...
I thought about making a pool toy. 

But, I thought I would have to weigh it down with quarters.
Then I saw ChrisONKey's pool torpedo and with some research
found out that if you up the infill % (in my case 90%) PLA will sink!

So I took chrisONKeys torpedo and with a little bit of OpenScad fu
made it so you can put some custom text on it :)
 

Print at 100% infill…. 90% infill floated!  But 100% sank
0.2 layer height and speed 
Also put a nice big brim on it to hold it down.