I have been swimming for about 8 weeks now. And although I have purchased a number of things to make swimming even more fun, I must say that the most important item to have is goggles.
Without goggles you cannot open your eyes underwater. And thus you miss many clues: how far you have left in the lane, if you are going in a straight line, and if something is in your way.
Now goggles are not a one size fits all sort of thing. Unless you already know what type and style fits you, please do not order them online. Most stores will let you open goggle packages so that you can fit them properly. This involves standing in the middle of the store and pressing the goggles to your face to see if they will stick for a few seconds. If they don't, they aren't right for your face.
I had to try on 3 pairs of goggles before finding the right pair and it has made all the difference in the world. So while I love my new *fancy* swim fins, and my new bathing suit, my goggles will always be my favorite swimming accessory.