The good thing about (something) is (clause).

Use this expression when you want to talk about the good points of something:

The good thing about my job is that I get a lot of vacation time.

The good thing about living in a big city is that you can get everything you need within walking distance. 

In grammatically correct, formal English, you should include "that" at the beginning of the clause:

The good thing about it is that it doesn't need to be watered that often.

In conversational spoken English, people often leave "that" out.

(something) needs to be (done)

Use "___ needs to be ___" to talk about work that someone needs to do:

It needs to be done soon.

English speakers use "be __" to avoid saying exactly who should do something. For example, you might say this this to an employee:

The files in this folder all need to be reviewed.

Instead of this:

You need to review the files in this folder.

The first one sounds softer and less direct.

Or you can say "___ needs to be ___" if you don't know who should be responsible:

That old building needs to be knocked down.

Use "___ doesn't need to be ___" when it's OK not to do something:

No, they don't need to be cooked. You can eat them just like that.

water (a plant)

When you give a plant water, it's called "watering" the plant.

Can you water the plants for me while I'm away?

You can also "water" a garden or lawn:

The best time to water your lawn is in the early morning.