Below is a walkthrough for the Alchemy: Miracle Growth side quest in Tiny Tina’s Wonderlands. To obtain this quest, speak with Wimarc in Overlands after blowing up the ocean.
Use potion on seaweeds
Interact with the seaweeds on the left of Wimarc.
Talk to Wimarc
Speak with him.
Go to cave
Travel south to the cave to begin the encounter. You’ll complete a total of 2 in order to get the potion from a chest. Once you grab it, enter the portal.
Talk to Wimarc
Speak with Wimarc again.
Pick up Sea Kelp Solution
Take the potion from him.
Use solution on poisonous seaweed
Interact with the seaweed blocking Wargtooth Shallows.