Of course it's possible: Your Mac is just a generic Intel machine (albeit one running EFI instead of a traditional BIOS unless you set up BootCamp). Grab your favorite BSD install CD, stick it in the drive, and have fun.1
1 - Because Apple is deprecating the optical disk you may have to make a USB install stick, or plug in an external CD/DVD drive.
Most of the hardware should work "out of the box", but you'll probably need to do some tuning. The FreeBSD folks have a somewhat outdated Wiki page with information on getting FeeBSD running on a MacBook, either under BootCamp or as the only OS on the machine.
I do have to object to calling OS X "Nothing more than a watered-down version of BSD" though. First off, it's Mach under the hood (the kernel) - they just borrowed the userland utilities from FreeBSD, and they've diverged pretty far since OS X 10.0.0 ; Second it's goal isn't to be a Unix development platform - it's a workstation.
Personally I find doing (unix) development on a Mac painful, and I wouldn't recommend it for serious, down in the muck unix programming, but as a workstation it's quite adequate (and you're not going to get a GUI like that from any of the open-source *nix systems without spending a pretty substantial amount of time working on it).
The amount of work required to get a useful desktop/workstation with any of the open-source *nix systems is substantial compared to running OS X. If you need a BSD (or Linux, Windows, etc.) environment any of the desktop virtualization programs can be used to run it on your Mac, while retaining OS X as the primary operating system.
You should also bear in mind that despite what some of us consider questionable design choices (I want to bludgeon the people who came up with the lousy disk I/O scheduling in OS X) Apple builds OS X to run on their hardware. It's about as optimized as you can get, and much like the Star Trek franchise, Even-Numbered OS X releases (10.2, 10.4, 10.6, 10.8) are pretty good.
So bottom line?
Yes, you can do it. No, I wouldn't personally recommend it.
There aren't enough benefits, and you lose a lot of functionality (or have to spend your time making things work, rather than being productive).