If a vessel is made from more than one piece, above or below the waterline, the joint or joints may leak. Leaks occur because the joint opens, possibly because its fastenings are being carried away or the stuff that was crammed into it to make it watertight is coming adrift. These calamities are generally caused by sudden stress on the hull, either by the hull's hitting something, such as a wave, the bottom, or another hull, or by severe stress being imparted to the hull. Pay close attention to your hull-to-deck and other joints, and do everything in your power to make them strong and watertight before you put your vessel in the water.
Some builders drill holes right through the hulls of vessels. And the truth is that some boaters give a higher priority to pumping the water closets and draining their sinks than they do to keeping the water out of their vessels. Perhaps they think they can do both at once. If you yourself are tempted to try these tricks, remember that it's dangerous business to drill holes in something that is supposed to keep water out.
If you drill a hole through the hull of your vessel and stick the pipe that drains the sink into it, make the pipe as strong as the hull, because you are now depending on the pipe as much as on the hull to keep the water out of your vessel. Put a valve in the pipe just as close to the hull as you can. You can use the valve to shut the sea out if the pipe should fail for any reason. Of course, the valve must be as strong as the hull. If you drill a hole through your hull, don't make it inaccessible by covering it with built-in furniture. Be able to reach those holes. Cut a wooden plug to fit every hole you drill in your hull, and keep the plugs handy. Be prepared to stop leaks. Be ready to plug holes you have drilled in your hull. Be prepared to stop water from coming in through joints that have opened up. Be ready to cover large holes created by catastrophe.
Waves can make it difficult to keep water out of a vessel. Driven by wind, they can become large and steep, breaking right over the top of the vessel, and they may become even larger and steeper and catch the vessel in their crest as they break, flinging it right down on its beam ends or, worse, turning it upside down. It's prudent for the seaman to study this phenomenon of wave and wind from the security of dry land. Study the Beaufort Scale carefully, using a complete copy that gives the time-honored descriptions of the numbered forces, words well used by seamen since time long before Beaufort, words such as fresh breeze and moderate gale. Note the verbal descriptions of the sea for each wind force; think about how your vessel would react to each wind force. A vessel will generally take more wind and sea than you think, which is A Very Good Thing.
Don't try to outguess thunder squalls. Assume every one of them is going to blow a living gale. Although strong winds can lead indirectly to water entering your vessel, there is nothing like a big wave for undoing directly and suddenly all your efforts at keeping the water out. Observe waves on all scales from mud puddle ripples to great ocean breakers. Watch how small waves combine and how waves run in from new directions. When big seas do this, they are most dangerous.