SO... what are boobs made from? Well there's fat, blood, mammary glands, and epithelial tissue for starters. We can get a somewhat decent idea of how dense that is, generally speaking. I don't really know how much of a boob is made up of each of those things on average, but let's presume that fat is the major component for this grossly unscientific model. Maybe I should have studied anatomy and medicine instead...
So... is fat less dense than water? I don't know, that's not my area of expertise. I would ASSUME that fat is less dense than water, but I'm no biochemist or anything.
So. AS FAR AS I CAN TELL... boobs float.
They'll float, but unless you have big, floppy torpedo tits they won't have enough leeway to go anywhere noticeable.
So you'd probably notice the boob rise a little less than an inch, ballpark figure, if you were lucky enough to observe your average lady in the process of lowering herself into a bath.
They won't act as floatation devices, or I couldn't have drowned half my victims.