Thomas Andrews made watertight compartments so water couldn't get in the boilers and sink the ship, it can deal with 5 boilers full of water and still float(p22-23). Not only that, but he didn't consider the water getting over the compartment though. The water continued to fill the compartments, after filling one, going over the wall and to the next compartment.(p22-23)
Also, he didn't put enough lifeboats on the ship, in fact he only put enough for about half of the shipś passengers!