Hey Kyler, good question. What I found interesting is you mentioned, "Sailboats," and not boats. Gnome is correct that most boats sink while moored because of some mechanical defect below the waterline, but this almost always applies to motorboats. Sailboats are a different story. The primary cause of sunken sail boats is swamping due to excessive healing in the wind, waves capping over the sides, or rolling over. You have to remember that since a sailboat uses the wind for propulsion, they are almost always out when it is windy, and that is also the only time waves form. John Vigor mentions in his book, "Everything I wish I knew about sailing," that almost all sailboats are not properly built to withstand a rollover without sinking. To do this the cabin space must be airtight, and the hatches must be closed and latched. Add to this the fact that almost all sailboat rollovers cause catastrophic damage to the rigging, which almost always renders the drive system useless, leaving the boat at the whim of all incoming waves until the boat eventually reaches the shore, or the bottom of the lake. I have been out many times on days when the waves were in excess of 6 feet, but we always close all the cabin hatches and latch any below deck access panels. Virtually every other boat I see during that time has people climbing in and out of the cabin door, which remains open the entire time they are at sea. In order for a sailboat to make use of the wind, it has to tip to whatever degree is necessary for the keel to come up on the opposite side of the sail and use its weight to counteract the force of the wind pushing on the boat. Tipping the boat far enough to take on considerable amounts of water simply becomes a matter of wind speed.
My son and I just finished a Lake Michigan crossing in our 23 foot sloop, not a journey for a beginner. We had a good weather window, and have sailed those waters for years now. The problem with many organized sailing events, like the Chicago to Mackinaw race is that they are put on a calendar, which is probably the worst way to plan a sailing trip. Basically race day comes, and the captains are told to either sail or quit, regardless of the weather. Many of them have invested the previous year in preparing for this one trip, they don't like to throw in the towel, even if they should. So each year it seems we end up with million dollar world class sailboats operated by professional crews sitting on beaches, rocky shoals or deserted islands. In some cases, some of the crew never return. None of these boats sink due to mechanical failure, virtually all of them simply tip far enough for one good wave to completely fill the cockpit and much of the cabin space with water in less than a second. Then the boat simply wallows like a half full bath tub out of control until the next few waves do it in. There are fantastic scenes in two movies illustrating this on very large sailboats, "Master and Commander," and, "White Squall." In the first, the boat was saved at the expense of one mast, and one sailor. In the second, despite the attempts of the crew, the vessel was completely lost in seconds and sank to the bottom. I'm not saying hollywood entertainment is the place to do your research, but as a sailor, I have great respect for the work put into the making of those two movies and the technical details that were included. The point to your question is simply that what sinks most sailboats is water coming over the sides, not through the hull.
I don't know of any good sources or websites to back up any of this, but if you did a search for sunken sailboats, you would find almost all of them went down in very bad weather and high winds. The Great Lakes are loaded with old Flying Dutchmen that tried to use the winds of November to make their destinations in record time, only to end up not making them at all. There are hundreds of sailboats on the bottom of the Great Lakes, only a small handfull of them sank while in the harbor. Take care Kyler, Rudydoo