Ships and boats are made to float on top of the water, but there are quite a few things that can go wrong to turn your boat into a submarine. Taking on water is inevitable -- large waves often break over the sides, and tiny leaks are common. This water will usually find its way to the lowest point of a boat -- the bilge area. For this reason, boats are equipped with bilge pumps to usher the water back out once it's reached a certain level. Boats often sink while docked, but unless you're like Sonny Crockett and you live on your boat, that's not a life-threatening scenario.
Common reasons a boat might sink at sea are:
Low transom -- The transom is the flat vertical surface that forms the rear, or stern end, of the boat. For outboard vessels, the motor is mounted onto the transom. For larger inboard vessels, you'll find the boat's name on the transom. The idea is for the transom to be high enough that it won't take on water. Sometimes, simple design flaws can leave your transom too low. Improper weight distribution can also lower a transom to the point that waves can come over it and flood the deck. To keep this from happening, don't store all your heavy gear in the stern of the boat. Scuba gear, coolers, fishing equipment and bait should all be distributed evenly along the ship to keep the transom at a safe height. You should also never anchor from the stern side -- it could pull the transom down even further.
Missing drain plugs -- This one seems like a no-brainer, but boats sink all the time because of missing drain plugs. When a boat travels forward, the entire vessel sits higher on the water than it does at rest, with the front higher than the rear. Water collected from waves or sea spray is allowed to exit the boat through a drain located at the rear of the boat at about deck level. Once you're traveling forward, the boat tilts up and the water will flow toward the drain and back out. The problem arises when the captain forgets to stop the drain once the boat is at rest with a small, watertight plug. When the boat stops moving, it sinks lower and begins to take on water through the drain. Carry extra drain plugs and try keeping one near the ignition as a reminder.
Cooling system leaks -- Boat engines are water cooled, pumping about 30 gallons of water through the system per minute for a 300 horsepower engine. If a hose bursts or isn't tight enough, this water can collect in the bilge and once again, you could find yourself sinking. Check for corrosion or obvious splits and breaks in the hoses and fittings of the cooling system before you depart. Replace anything that looks suspect, and you should be fine.
Navigation error -- Simply put, this means striking an object with your boat. It could be rocks, ice, reefs, logs, or anything else large enough to do damage to the hull, or body, of your boat. The best way to combat this is by being careful. Slow down if you see debris and be especially cautious after storms, which can wash in a great deal of foreign objects. If you see something floating, there's a good chance there's more under the surface. If it sounds like you've hit something, stop the boat immediately and check outside and below for holes or leaks.
Stick that plug in the drain and click forward to read about what safety equipment you should have on board.