I almost touched a sea urchin when I first went diving, didn't know what it was. Apparently they are very painful. That and the sea snake. The sea snake fkn freaked me out (I have a snake fear), I would have beaten Phelps in any swim race.
per wiki Some have gentle dispositions and bite only when provoked, but others are much more aggressive. Currently, 17 genera are described as sea snakes, The only reason I know this much is because it recently sparked an interest.