if you've done it right then you should simply use a rubber seal like, once again, every other water block uses.
There is this stuff (sorry dont know the name) that I'm sure you could get at your local pet shop that has aquariums, pumps and things for fish and crap....it's a gel-like substance that comes in a tube thats made to put on the edges and corners of aquariums to seal them from leaks...it comes out a gel and then kind of solidifies...not like JB Weld solidifies as it's still kind of rubbery but it makes the seal.