Questions & Answers about chuán zài shuǐ shàng.
在 is the word that marks location here. In this sentence, it works like at / in / on in English.
So the basic pattern is:
Thing + 在 + place
- 船在水上。
- The boat is on the water.
In other sentences, 在 can also act as a main verb meaning to be located at.
Chinese often uses a localizer after a place noun to show the exact spatial relationship.
Here:
- 水 = water
- 上 = on / above / on the surface of
So 水上 means on the water or on the surface of the water.
This is very common in Mandarin:
- 桌子上 = on the table
- 椅子下 = under the chair
- 房子里 = in the house
English often uses just one preposition, but Chinese frequently uses place noun + localizer.