Learn where current is likely to be unusually strong or unpredictable, such as around corners, pinnacles and walls. Don't reach where you can't see. Just remember that any hole is likely to be somebody's home. Though you might not touch the creature, you may touch its defensive perimeter and provoke an aggressive reaction. Don't wade barefoot.
Sand often hides dead, but still sharp, coral.
Stingrays often lie hidden under the sand in tropical areas and even as far north as Cape Cod and British Columbia. They have a sharp spike on their tails and will attack if stepped on. Wear sandals or booties, shuffle your feet and probe the bottom ahead of you with a stick. It's controversial in some areas where divemasters feel that gloves increase the temptation to poke and fondle delicate marine life. Sometimes headache, chills and fever.