1976 laser

I just got a 1976 Laser and I’m sure its a little soft and prob had repairs but for a couple of bucks it might help me learn to sail. I have had it out three time and I made it back to the dock all three times. It has also captsized and it rights up pretty easy. I was looking at parts as I need a new hiking strap and saw that plastic bottles for air are sold as a replacement for styrofoam. Looking through the inspection port I do not see anything to aid in floatation. Do I need these bottles? Will the boat sink without them? Thanks for your help.