Hi, should I use "to wait" or "waiting"? Why?
It's a mere waste of time waiting any longer.
It's a mere waste of time to wait any longer.

It's a waste of time watering the garden when it is raining.
It's a waste of time to water the garden when it is raining.

Which sentence is correct?

Question

Hi, should I use "to wait" or "waiting"? Why?
It's a mere waste of time waiting any longer.
It's a mere waste of time to wait any longer.

It's a waste of time watering the garden when it is raining.
It's a waste of time to water the garden when it is raining. 

Which sentence is correct?

Answer

Rabbit, 

All of your sentences are correct. 

The gerund 'watering' and the infinitive 'to water' both serve as appositives for the subject 'it'. Both the gerund and the infinitive function as nouns that restate the subject 'It'. When they are placed at the end of a sentence with 'It' as subject they are called delayed appositives. 

It's a mere waste of time waiting any longer. = Waiting any longer is a mere waste of time. 
It (waiting any longer) is a mere waste of time. 

It's a mere waste of time to wait any longer. = To wait any longer is a mere waste of time. 
It (to wait any longer) is a mere waste of time. 

It's a waste of time watering the garden when it is raining. = Watering the garden when it is raining is a waste of time. 
It (watering the garden when it is raining) is a waste of time. 

It's a waste of time to water the garden when it is raining. = To water the garden when it is raining is a waste of time. 
It (to water the garden when it is raining) is a waste of time.

Answer

The -ing form of the verb (gerund) usually refers to an activity in general (It has the characteristics of a noun) The infintive refers to an action and has the characteristics of a verb. Both can be used as the subject or complement of a sentence (unless other rules dictate specific use like gerunds follow prepositions etc) but in general the gerund sounds more natural and the infinitive sounds more abstract.  If you turn these sentences around you will hear the difference. 'Waiting any longer is a waste of time'. 'Watering the garden when it's raining is a waste of time'. The gerund is more natural here and more common.  If you are confused just remember that 90 percent of the time we use the gerund as the subject or complement of a sentence.

Answer

waiting and watering
Learn the following:
It's a waste of time watering the garden. 
It would be a waste of time to water the garden.

These are the correct forms. There are reasons, but they are quite subtle. See if you can work them out for yourself.

Answer

to do and ing are both correct in your examples. And actually no big difference.