system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why don't boats have transmissions like cars do? Wouldn't that make them be able to go faster?
gpt
Some boats, very high performance ones, do have transmissions. But for most normal boats, this adds a lot of complexity and, most of all, cost.

The other factor is that water behaves very differently than air. Water resists being pushed out of the way, and the harder you push, the harder it pushes back. The fancy term is compressibility. It's because of this that a fall from a great height into water tends to be like hitting concrete. 

So, back to the boat. As you spin the propeller faster and faster, it gets harder and harder to turn. Also, the drag of the water against the hull increases, requiring more effort. So, a boat in water is like a car that is going up a very steep hill. It needs to gear down to maintain speed or accelerate.  With a boat, it works out that most of the time you only need one or two gears.