diatomic == di-atomic == two atoms bonded == He2
http://en.wikipedia.org/wiki/DiatomicHelium is an ideal/inert gas, so, truthfully, He2 is not really stable - you have to pop some serious energy into it to get it to form, and it will quickly decay from that state back to a lower-energy (2)He (unbonded atoms). H2 is what was in my head, but then I though, nobody uses that anymore (at least since the Hindenburg!), so it would be a poor reference.
I should've picked a different (and better) example. H20 is a much larger molecule/structure than a simple He atom. Nevertheless, I think the explanation still stands and is completely valid.
I'm sure the mechanical engineers immediately thought of a mechanical seal, not unlike, say, the rear main oil seal on the crankshaft of your car (though neoprene pretty much rocks for all sorts of mechanical seals plenty of which are "watertight"). Because Water (H20) and oil (hydrocarbons - C(n)H(2n+2)) are much larger structures, they don't seep through as quickly.
The real point of all this is: the bouyancy of the wetsuit is due to the air bubbles in the foam. The same properties make it susceptible to absorbing moisture. The older the suit, the more it will absorb, due to UV damage (breakdown of the neoprene) and mechanical breakdown (the little bubbles get broken open).