I was told about Jonah one day wilst adventuring in the Lake. Being a level 17 Druid and Jonah not being a tree that I could hug, I decided to kill him. He conned blue to me at this lvl. I would not fight him in the water as the water has sharks. What I like about him is he spawns fairly quickly, usually just after I recover from the last fight with him. Water is the only loot I've seen on him.
I sit in the back corner of the cove and pull him there. Bash him till he starts to run away and then root him. I've lost him once running away when he dove into the water.
Happy hunting!