does salt and water taken immediately prevent pregnancy?

No, the only way you might be able to prevent a pregnancy is to take the morning after pill or Emergency contraceptive pill which needs to be taken within 72 hours. The sooner it's taken after sex the more effective it is.

If you are going to be sexually active and do not want to get pregnant, then you should consider getting yourself on some regular birth control that will help prevent pregnancy but will not prevent STD's.