I couldn't find an explanation of why one can't just put a real giraffe in a tank, fill it with water and "see what happens". (Obviously I suggest doing this in such a way that one can quickly rescue the giraffe in case it's not buoyant after all.)
It makes complete sense to me that most land mammals have at least rudimentary swimming capabilities, because it's probably selected for. Not very often, of course, but whenever there's a flood, all non-swimming animals in a region die at once, which is a lot of selection pressure.
I guess the trouble and expense outweighs the curiosity. In particular, I bet the intersection of people who have a spare giraffe and people who don't mind an ASPCA/PETA protest is very small.
Another example of an easily testable issue: Do people swim faster or slower in a more viscous fluid? Newton and Huygens, among others, argued back and forth over this, but no one bothered setting up an experiment until 2004. The paper went on to win an Ig Nobel Prize. (Answer: Viscosity doesn't seem to affect a person's swimming speed.)
Along the same lines, the Mythbusters had an episode about swimming in syrup. Here's a link to the first part of the episode: http://www.youtube.com/watch?v=Hs4Q5jZSJbA
He says: "In the case of the question "Can giraffes swim?", we just aren't able to use real giraffes, so our approach is - at the moment - the only one we can use to test it."
That made me laugh as well. I mean, you're right why don't they just put a giraffe in a tank? Of course that brings to mind imagery of a 19th (or earlier) century scientist that would likely not have any care for the animal's wellbeing.
I'd imagine it's a combination of two factors. Firstly cost and secondly public image. I'm guessing a giraffe tank would be quite unjustifiably expensive and I could well imagine a university funding board rejecting this idea purely from the bad PR potential of scientists killing of giraffes rather pointlessly.
It makes complete sense to me that most land mammals have at least rudimentary swimming capabilities, because it's probably selected for. Not very often, of course, but whenever there's a flood, all non-swimming animals in a region die at once, which is a lot of selection pressure.