Steps to reproduce:
1. Download and open project "BuoyancyEffector2DBugTest.zip"
2. Open and play scene "level01"
3. Notice that smaller quad has "Auto Mass" off and "Mass" set to 3. Bigger quad (the one with buoyancy) has density of 2

Actual result: Smaller quad sinks
Expected result: When "Auto Mass" is of the smaller quad is set to off, its density supposed to be 1, therefore it should float (but instead it sinks, because mass is used instead of density)

Note: This is based on the documentation description in http://docs.unity3d.com/ScriptReference/BuoyancyEffector2D-density.html

Cannot test for regression on 5.2 or below, because BuoyancyEffector2D did not exist prior to 5.3

Tested and reproduce on:
Windows 10
Unity 5.3.1p4, 5.4.0b3