Alright kiddo, so when we talk about the "head" on a watercraft, we're actually talking about the toilet!
Just like how we have toilets at home, boats and ships need them too so people can go potty while on board. The head is usually located in a small room or compartment on the boat, and it has a special bowl to catch the pee and poop.
But unlike the toilets we have at home that use pipes to flush everything down, boat toilets use water to push everything out into the ocean or river. They do this using something called a "pump-out" system.
Basically, there's a tank under the toilet that collects all the waste, and when it gets full, a special boat called a pump-out boat comes alongside and sucks all the waste out. This way, the boat's waste doesn't pollute the water it's sailing in.
So, that's what the head is on a watercraft - a toilet that uses water to flush things out and keeps our water clean!