One of the most loathed creatures by swimmers, leeches are well liked by fisherman as they make decent bait.  They do suck blood, attaching by a sucker around the mouth parts.  Their saliva contains an anasthetic and an anticoagulant so the victim doesn't feel the mouthparts enter and so the blood doesn't clot.  When a leech has had its fill it will let go, but they can be removed by burning with a match or with hot tap water.   Their grip is quite strong so don't be tempted pull on it, their slippery skin makes it hard to hold onto them and grabbing and pulling is often counterproductive.  Leeches are excellent swimmers, they swim in an undulating pattern.  They are commonly found on the flesh and legs of snapping turtles.