As a lifelong swimmer and former coach, I can tell you that the most important piece of equipment for swimming is the goggles.  I would encourage anyone from beginner to advanced to invest in a good pair of swim goggles.  They can help your visibility immensely whether you are swimming in a pool or in a lake.  Goggles really help beginner swimmers because you are able to watch your stroke underwater and make corrections if you need to.  For the advanced swimmer, goggles help with visibility in seeing the walls for your flip turns.  They can also help you see other swimmers in a race!  So, if you swim, get some goggles.