There are so many reasons a boat can sink. Larger boats can lull you into a false sense of security. The bigger the boat the more things can go wrong. Just read some of the boating disasters that have happened over the years. http://en.wikipedia.org/wiki/List_of_maritime_disasters or http://stevesmaritime.com/mbang.html.
A really great place to look for accedent reports is http://www.ntsb.gov/investigations/reports_marine.html Go there often and see what can and does happen.
Why Boat Sink is from the Seaworthy Website
The number one defense for sinking is to have a working and redundant bilge pump system which may include an emergency high volume auxiliary pump. High water alarm is also a necessary piece of equipment. Additionally, a bilge pump monitor is required. After all if you don;t know the condition of your bilge pump system you don’t know if you are safe. See the Helmsafe products.
More reading about keeping your boat afloat