I think the idea of pressure that you have is probably not correct. We know that the DO is easily powerfull enough to destroy reality itself, so to use your analogy, there is likely more than enough water that opening the bore fully will simply increase the rate of water flow and flood the world even faster.

Currently, the bore is thankfully small enough that the DO can only influence the world to a limited degree, but the bigger the bore, the worse the effect on the world. Breaking all the seals would not free the DO, but it would certainly allow the DO to touch the world more openly, which is not good. It would also allow the DO to once again start enlarging the bore and coming closer to full freedom. Not a good thing.

It should also be said that the War of Power lasted only ten years, and the people at the time were likely far more capable of resisting/surviving the DO's attacks both due to their advanced technology and their advanced One Power capabilities. The people of the third age are not so lucky, so the fact Rand wants to break the seals without a good plan is even more bizarre.