They come to that conclusion, too. Cue some very cold twins sailing away with haste. 

They warm up quick enough once away from that island.

The next island they see is a smallish spar of rock with a huge shell of some kind, as tall as a house, sitting on top.

"Cooool." 

They decide to explore it.

The big shell has an entrance that is boarded over by thick wooden beams.

Also, a new version of the chu-chu monsters appears: Blue. It gives off electric shocks!

Is there anything they can throw at it?

Small rocks. It seems to have a recharge time, too, they could watch it for that.

They wait for it to give out a shock, then beat it to death. 

It drops a chunk of blue chu jelly. Seems like that was the only monster here though.

They get to battering down the wood.

The wood is fairly resistant to breaking down. They may have to light it on fire.

Tabitha fetches the fire potion. 

It burns quite nicely, doesn't it?

Yup. What's inside?

A large hollow area, walls in blue seashell motif, with a few tiny fairies flitting around near the ceiling, looking down at them nervously.

"Um, hi. Do you speak whatever language this is?"

Apparently not.

They feel watched. By something other than the tiny fairies, that is.

He sighs. "Is there a big monster in here?"

"Excuse me, young ones. I am not a monster."

She was there all along, just in the corner of their eyes.

"Oh, hi. Are you a ghost?"

"I am a fairy. Why are you here?"

"We wanted to explore the world and find some friends to make like us."

"The fairy fountains are meant to help those in dire need and heroes of the land. I do not blame you for curiosity, but since you are neither, please leave this sacred place."

"Being heroes sound fun. Can we try that?"

"I cannot help with that. Oppose evil at every turn and eventually you will qualify as a hero."

"We'll try. What do these springs do?"