I am the father of four. All girls, (god help me) two of which are teenagers, 17 & 15 plus one 11 and one 3 (don't ask). I have some experience here.
No. 1: Who paid for the system? And honesty is the best policy here. It may look cool to say that you did, but if your parents really did, then I can see their concern about pumping virtually thousands of gallons of water through what they probably paid several hundred dollars for.
No. 2: Why? What is wrong with the system the way it was originally? Why is all of this extra cooling necessary? Are you overclocking just to see if you can? If that's the case, then maybe you should re-think your motivations. This reverts back to number one. If you have a leak severe enough to damage the machine beyond repair, your parents may not be able to afford to replace it. Frankly, I am building my first o\c machine. I have good reasoning behind me: I am currently running a PIII 450. While it is fine for many applications, it will not run many newer programs. To avoid another costly upgrade in the near future, I am building a machine that I can overclock to a higher speed at a lower cost than the same machine with the higher specs. I cannot afford to buy a 2.8G machine from a "dealer", so I am setteling for the next best thing. Building an o\c machine is cheaper, in other words.
(are you getting the subtle hints here?)
No. 3: If you're parents use the same machine, maybe they are concerned that a disaster would leave them without a computer.
Summary: If you paid for the machine yourself, I don't see any reasonable parent objecting to your desire to water cool, unless of course you intend to cut holes in the walls of your bedroom and tie into the bathtub cold water line. However, if your parents paid for it, thank them, love them, and honor their wishes.