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.
The yutyrannus, not the rex, is your best friend for leech hunting. The leech is vulnerable to the fear roar, and the debuff reveals their location. This makes hunting them a simple errand. You'll collect so much leech blood you could start a swamp fever clinic.