Übergeek 바둑이:

Case 1:
The police have caught a thief suspected of a string of
robberies at convenience stores. Since he has refused to confess to his
crimes during interrogation, the police decide to waterboard him to
elicit a confession. He confesses to his crimes and goes to jail.

Waterboarding should not be used to obtain confessions to a crime.  No lives are at stake here.

Case 2:
A
suspected serial killer has refused to confess to his crimes during
normal police interrogation. The police decide to use waterboarding to
make the serial killer confess and disclose the location of dead
victims that have not been found by the police. He confesses and is
sentenced to death.

Same as above

Case 3:
A child serial rapist has
abducted a child. If the police do not find the child it could die. The
police caught the suspect and decide to waterboard him. The suspect
discloses the location of the child and is sent to jail.

Yes, water board the pervert and then let the crowd beat the living crap out of him (after you accidently leave his cell door open).  Some jerk knows the whereabouts of my granddaughter I'm going to start taking off fingers. 

Case 4:
A
communist agitator has been organizing workers to join unions. He is
also suspected of organizing demostrations against the government. The
military capture this man and waterboard him to make him disclose the
location of his associates. The man confesses and he and all his
associates are sent to prison.

Same as number one.  No to waterboarding.

Case 5:
A man is supected of
being a member of the communist party and organizing guerrilla
operations against the government. He is captured and waterboarded to
make him disclose the location of the guerrilla command and all of his
revolutionary comrades. He confesses and is sent to prison. His
comrades were never found.

It depends on whether or not there is an immediate dangers to innocent people.  If you join a guerrilla organization intent on harming the US, and you get caught, don't expect tea and crumpets. 

Case 6:
An radical anarchist is
suspected of carrying out bombings against banks. This has disrupted
the businesses of banks and cost millions in property damage. He
confesses to his crimes during waterboarding and is sent to prison.

No.  Same as case 1

Case 7:
A
man is suspected of being a member of Al Qaida and of having
information in a bombing that could leave hundreds of people dead. He
is subjected to waterboarding, but refuses to confess claiming that he
is innocent. During the course of investigation it is found that he is
indeed innocent and he is set free.

Yes to water boarding.  It's not like they just would randomly pull some guy out of a crowd.  There was good reason to suspect him.  You can't know he's innocent until after a full investigation.   But when lives are at stake, you do what you have to do.  Waterboarding is simply unpleasant.  For some, a trip to the dentist is worse.

I divided my cases as follows:

Case 1: a common criminal
Case 2: a dangerous criminal with no victims in imminent danger
Case 3: a dangerous criminal with a victim in imminent danger
Case 4: a political prisoner not implicated in acts of terrorism
Case 5: a political prisoner suspected of acts against the government
Case 6: a political prisoner commiting acts of terrorism
Case 7: a suspected terrorist who is later found innocent

So my questions are:

If
waterboarding is not torture, why are the police and other law
enforcement agencies not allowed to use it when interrogating prisoners
(cases 1, 2 and 3)?

Because one of the arguments for water boarding is that it is to be used only as a last resort and only when there is good reason to believe that lives are at stake.  I have no sympathy for people who want to kills us.  A bank robber, once caught, poses no more threat.

Why is waterboarding not used to put extremely dangerous criminals away (case 2)?

See the previous answer.

Is waterboarding acceptable to save a human life (see case 3)?

Of course it is.  So is removing some dudes fingers.   Maybe a few other parts too. 

Is waterboarding acceptable for undesirable political views or political prisoners (case 4)?

No

Is waterboarding acceptable for any military action involving guerrilla warfare or insurgents (case 5)?

If lives are at stake -- yes

Are all forms of terrorism (including mere economic terrorism) a good reason to use waterboarding (case 6)?

If lives are at stake, yes. 

What do we do is somebody is subjected to waterboarding and is later found innocent (case 7)?

Buy him ice cream.  Let's face it, they don't pull nuns out of the convent and water board them.  If you join a terrorists organization, and then are suspected of doing something that jeopardizes the lives of others, don't complain when given a bath upside down.  Here's a concept:  get a good job, don't join a terrorist group bent on hurting others. 

I
find that waterboarding is one of those areas that some people see as
black and white, and other see as grey. I am curious to see some
opinions.