RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Bowl of water
The Old School RuneScape Wiki also has an article on Bowl of water.

A Bowl of water is made when using a normal bowl on a water source. A bowl of water cannot be used to counter the desert heat effect as the water evaporates under the heat.

You can make an incomplete stew by using a potato on the bowl of water. Bowls of water, just like buckets and jugs of water, can be combined with pots of flour to make dough.