As crazy as it sounds, they intentionally put holes in your boat.....below the water line.
So, you might ask why they do that since it sounds like a really "stupid" idea, right?
An under water thru-hull is used to allow water to come into the boat and for "stuff" to go out of the boat.
Things leaving the boat are generally "waste products" such as your poo. There are very strict laws governing when you can dump waste and violating these laws and getting caught will likely get you fined. Poo is called "Black" water. How to set your boat up to be legal if you can dump black water directly overboard is a whole different article. Water you use to wash your dishes is called "Grey" water. Gray water can normally be dumped directly overboard everywhere, but check local laws.
Water coming into your boat is generally used to cool engines. Motors & Generators and Air Conditioning will be called "raw water cooled" if your boat uses under water thru hulls to keep things cool.
These connections feature "raw water strainers" which assure that the water being used to cool your engine(s) is clean. The water comes into your boat through what looks like a clam shell that is stuck to the hull. Under the clam shell, is a pipe molded into the hull that is connected by a hose to a raw water strainer. The pipe is "melded" into the fiberglass or welded into the metal hull. These connections are very secure and rarely fail, it's more common that the hull itself fails than these connections. With that said though, these connections require attention because the hoses between the thru-hull and the raw water strainer cause problems over time. The thru-hull will have a ball valve with small handle that allows you to turn on or off the flow of water.
The hoses should be checked for integrity every year and "replaced" every 7-10 years, they should also be "double clamped" to the nipples of the thru-hull and the strainer. Think about the pressure that exhibits when the thru-hull is several feet under water and then think about the increase in pressure that occurs when the boat is underway. If that hose comes off, you're probably going to sink before you can turn the ball valve off. The photo shows a hose that has split due to age and is missing its 2nd clamp. Also shown, is the handle to the sea cock.... (ball valve) that needs to be checked to ensure it can be turned on and off.
Water comes into your boat from whatever type of reservoir you're boating within..... fresh water lakes, brackish water or salt water in the ocean....water is pulled into your boat to the engine via the raw water pump (impeller). After going through the strainer, it is pushed into the engine block at the thermostat housing. The thermostat prevents your engine from experiencing shock. For example, you're boating during the winter and the lake water is 33 degrees Fahrenheit. Some of the water waits until the engine is up to temp and then after the engine is warmed, flows through the block cooling your engine. The other part of the water is routed to the exhaust manifold to cool and silence the exhaust. That is the basic outline.
Your transmission is cooled by raw water prior to the impeller pump.
Yes, your engine does have oil inside also, just like your car.
So, I hope that helps you understand why there are holes in your boat.
