Tamable if you knock them out. But it is extremely difficult to do as they have low health. You have to feed them rare flowers and it is an instant tame. Watch out for the diseased ones as you cannot knock them out.
If you're looking for leeches (to get leech blood) just listen, cuz they make a sound in the water when you're close to them (it sounds similar to someone making but bubbles inside the water). Everytime I go to the swamp i always find them quick and easy due to this and I hope this help other people too.