well first you have to find the cave then when you think the monster is gonna strike jump and hit his head a sleep for a while but will eventually wake up and after all five heads are down go grab that scale :)
In the underwater cave.
underwater cave
It's an underwater cave at Poseidon's "castle"
you dive down underwater and you go into a cave
You have to swim through an underwater maze. Pop bubbles for oxygen.
underwater
The Hydra is in his underwater lair. Swim out from the beach to the underwater maze, then go right, down, and left to find the Giant Pearl and the entrance to the Hydra cave.
if you go into poisedon's little beach thingy, you go past the two gods, and swim to the far left side of your screen. There will be a dark spot in the water and click on it and you will go diving. It took me a couple of tries to find the way but you'll get there. eventually.
a cave that is underwater in freshwater
the cave will have the minitours face on it
Use Dive.
by precipitation