
The areas outlined with red are marshes.
While traveling through a marsh, your character will move at walk/crawl speed only.
To find leeches, you basically do what you'd do IRL: you drop your clothes and go commando wandering the marsh. Just standing there bare-ass will work too. Every free slot on your character doll will sooner or later be occupied with a leech. If you don't want to lose soft hitpoints at the marsh, you can gather a leech into your inventory and reapply it later, when you're safely inside your base. After 3 ticks of SHP drain Leech will become bloated and after a while in a container bloated Leech becomes hungry again. Each SHP tick raises Leech's potency by 1, softcapped by your CON.
For later use, to determine the biome you're looking at, hold CTRL+SHIFT and mouseover a tile (avaliable in Amber or similar custom clients). If you don't see a tooltip for the biome, press SHIFT+i once to turn tooltips on.