I think that when the whirlpool opens it should function sort of like to Enchanted Springs, and you should be able to send a pet down there in a submarine, or something.  You could find things, like in the Springs, but they should be all deep ocean themed, like a bottle of giant squid ink or a fossilized trilobite.  Also, there should be a slight change to get an effect for the creatu you sent down there. I think like a "deep sea" effect, would look good. maybe the creatu is deep blue, with glowy little things along their back, and fins and maybe an angler fish light!

In my opinion that is way too similar to the Springs, especially since they're on the same planet. The "Deep Sea" effect you were talking about sounds too similar to Frostlight, with the deeper colours and glowy bits. ^^; So no support on that part.

Heres my idea i made for two years ago: 

Wirlpool in Atquati

In this event a couple of pirates need
help from you to find a treasure. They want to take you on a long
trip around atquati, to follow a map. But before you can go on this
journey, you have to get a pirate saber, some food, and rum! When you
are traveling you suddenly see the water moving, and a big wirlpool
opens up beneath you. Then you get sucked down in the underwater
world. The residents need help to something. (needs more info)

I like the 'effect' idea.