It really depends on the type of saltwater aquarium you are planning on having. If you plan on a reef or fowlr, then, in most instances you don't need any other outside filtration, except for a protein skimmer. The live rock and sand becomes your bacteria filter. I guess you can do without the protein skimmer, but most people will advise you to use one. I would and do.
You can run a power filter if you want to, but you don't really have to. I don't have one (at least, I can't find it and I wanted to use it today-changed some water and stirred up stuff). If you do use one, be sure to clean the filter every two-three days. If you want to use chemicals/carbon, etc, then having one around might be a good idea.
Hey, good luck.
Frank