Are we supposed to use Water Pills after liposuction surgery? Tummy Tuck Surgery?

I do not recommend water pills after any surgery. It is perfectly normal to swell and retain fluid after the stress of a surgical procedure. That is how the body responds to stress and heals naturally. The extra fluid will be mobilized and the swelling will go down. I do not recommend anything that interferes with the body's physiology as the risk can outweigh the benefit. I would follow all your plastic surgeon's instructions to avoid problems and avoid the tendency to "self treat" based on advice from a person's  anecdotal experience. 

Well, some surgeons recommend them and some don't' it's a matter of opinion, neither is wrong or right.  I myself do not recommend water pills.  They are just one medication thrown into the many that you are probably already taking (antibiotics, pain pills, sedatives, etc., etc.,) that I don't see the benefit.  The swelling that the water pills are supposed to help reduce goes away by itself progressively.