Hey Andy (don't you just HATE people call you that??

)
As for the whole 'melamine is waterproof' thing, well, if it is the kind of melamine that you buy from bunnings, as long as you seal the corners w/ silicone, as far as I know, it's pretty water proof.
BUT - if it's not THAT type of malamine, but more an MDF with a plastic-y coating on it, then it WILL absorb moisture and will eventually bubble!! (I have some tanks that were sold to me as melamine that have done this!!)
Good luck hun
Bex