Well the good thing about it is you don't have to renew the license every couple of years so once you've got one it's there for life. As far as diving goes the more you practice the better you become and when you're away from the water you kinda lose the skill and you might lose the basics you spent days learning. If you have never been to the ocean before then perhaps you should visit it first and become comfortable with it so you do not panic when you dive into the deep for the first time, it can be overwhelming and I have heard stories about first time divers that took pool diving courses that panicked when they hit the real thing.
Scuba is not confined to salt water, you can dive fresh water too but there isn't much to see because the water is not clear but that depends on the water source in your area but it will give you a clear idea about the process.
If you want to try diving there is an intro dive available at most dive centers which is just a trial dive, you don't need a license for that but I believe a diving instructor will be holding on to you from above or something of the sort.
