There's nothing quite so frustrating as missing an entire day's diving because of something inane like breaking a fin strap and having no spare. It doesn't take much effort or investment to make a spare parts kit and with it, you minimize the probability of missing dives due to minor problems like a broken fin strap

You make a spare parts kit by collecting those sundries that war out, break or vanish at the worst time, and storing them, with a few basic tools, in a moisture proof container in your equipment bag. At first you won't need much room for this, but as you again experience, you'll add to it never throwing anything away until it's basically and equipment locker you need a fork lift to move. But that won't happen for a few years, so here are a few suggestions to get you started :

  1. Mask strap tip : fabric/Velcro type straps fit virtually all masks, making them "universal",
  2. Fin strap tip : when one goes, the other's close behind. Carry two and replace them at the same time,
  3. O-rings tip : different tank valves take slightly different sizes ; carry and assortment,
  4. Silicone lubricant tip : carry silicone grease, but spray and use it very sparingly according to the manufacturer or the particular equipment. A small container will last a decade or more, or until you lose it,
  5. Snorkel keeper,
  6. Cement for exposure suit repairs tip : different suits require different cements,
  7. Waterproof plastic tape,
  8. Quick release buckle, 
  9.  Pocket knife, 
  10. Pliers tip : even better, a plier-tool, like the leatherman tool,
  11. Adjustanle wrench, 
  12. Spare sunglasses, sunscreen ( will sealed so it doesn't goop up your kit), motion sickness medication. ( these aren't spare parts, but things you really don't want to be without so make them a permanent part of your kit).

Your instructor can suggest other items for your spare parts kits.