I love this question. The best answer would be any of the following:
The flowers need to be watered. (This is what you would say to someone else about the needs of the flowers -- a necessity for the flowers to survive.)
The flowers want to be watered. (This is almost as though you are speaking for the flowers, expressing their desire to be watered.)
You could also say, "The flowers need watering." Although this suggests that you are looking at the flowers and observing their lack of water.
There are other variations:
It is important to water flowers.
It is important to water the flowers.
Flowers need water, and that's why I water them.
Flowers want water.
Flowers want to be watered.
Flowers need water.
Flowers need to be watered.
I hope this is helpful!