The meaning of mu;hii:t is 'ocean', and the water in clouds comes only from the ocean. In this regard it's very fine to tell the clouds to come and drink water. That is, if you want to confront/examine my weeping, then first obtain the water from the ocean of my tears. Or again, it's only advice-- that no matter how moist the cloud might be, it cannot equal my weeping; compared to my weeping, it's dry. First let it drink water and become wet, or become full of water, then let it claim to equal me. There's also the aspect that the cloud is being invited to drink water-- that is, the message is being sent to the cloud, 'if you want to drink water, then come'.
The meaning of kisii ko paanii pilaa denaa is 'to cause injury/shame to someone'; and paani par basar karnaa means 'to live in straitened circumstances'; in the second line is an allusion to both these idioms. The suspicion arises that paanii piyo to aa))o might be an idiom like mu;Nh dho kar aa))o , but it's not found in any dictionary. It's possible that from paanii pilaa denaa Mir might have made his own idiom-- that if you want to 'drink water' (that is, to suffer injury/shame), then come along.
