When I was in Molokai Hawaii, I was walking around a coral reef area called, "20 Mile Marker." I was eight years old, btw.  While I was turning over rocks looking for sea cucumbers
, I saw an orange sea slug
.  I went to pick it up but when I did I felt a sharp pain (sting).  I dropped immediately and picked out little white needles from my hand.  Well, I immediately walked out of the water and a local came over and told me, and I 1,000,000% agree with him, "NEVER go walking in the water bare footed." >___> Well, that wasn't the case for me.
So, I go back to my Mom and sister, my Dad had left to go back to the main land, but I didn't tell anyone.  You know, 'cause I was eight and all... worried about getting yelled at. >_>  The next day, my thumb started to swell up, and I mean SWELL.  It was read and looked bruised.  My Mom treated it but we had to go to a doctor.  Good thing he/she, I forget, was able to cure it beacuse if I had waited another 48 hours, I would've had blood poisoning.  But, luckily, that didn't ruin the one month vacation at Hawaii.  Later, from asking my Mom's friend who lives in Hawaii, I was stung by a sea centipede.  I have another story like this but it involves being in a cove with giant boulders of coral and strong currents.
So remember kids, don't do drugs or pick up anything that looks unusual from the water.