The only way to the temple of the swamp is by boat. There is one right there. Convenient!
Swiggity Swooty, he comin fo dat booty.
Welcome to Underowl Cavern, an underground sanctuary lost in time where the Underowls used to gather and prepare their ceremonies. A clue to Link's quest is said to reside within the stone and dirt walls... (Yes, those are functional Pegasus Boots, and no, this is only a Proof of Concept shot).
It's the harsh marsh located right of bridge ridge.