Water gets in there when it splashes into the boat or when something bad happens like a leak in the hull or you forget to put the plug in at the launch ramp. Ship or boat owners install bilge pumps at the lowest possible level of the ship below the sea surface in an area called the bilge.

Sunken Ride The Ducks Boat Had Reduced Bilge Pump Capacity
How does a bilge pump work on a boat. Bilge pumps are essentially electrical water pumps that are wired into the ships main electrical system or to a marine battery. Its just a pump for getting water out of your bilge. It is even more important in an emergency when you are trying to reduce the amount of water your boat might start to take on. This is vital for ensuring that your boat can stay afloat. The main job of bilge pumps in many boats in clearing different nuisances in the water. There are pumps bought together with the ship and with time they end up wearing out and in such cases you will need to purchase a new bilge pump.
The primary job of most bilge pumps on most boats is to clear nuisance water from the bilges packing gland drips spray from an errant wave etc. The pump will help you in determining whether there is any licking taking place in the boat or not. It comes on automatically when it is submerged it has a float switch in it. The bilge forms the joint where two sides of a ship or boat connects. It can also be turned on manually via the dash. The main goal of a bilge pump is to clear out water from the bilge area of your boat.