It's easy to step on sea urchins. Many of them are less visible and much smaller than you see in photographs They can be found at the reef at low tide, 20 cm across the main body.
It's painful to get stung and takes a long time for those wounds to heal, so prepare yourself in three ways:
1) look carefully when wading or putting down your feet.
2) wear sandals that attach well to your feet in water.
3) bring some vinegar to apply to sea urchin punctures - vinegar dissolves the broken needles in the foot.