So I bought all the pieces, I have no clue how to hand it in :(

Go to Games > Treasure Maps > Unknown Island

thanks!

I'm just interested.. What did you get?

^ Once you complete the map, you'll meet Merana. Complete her quest and she'll turn one of your pets Hydrus.

Besides Merana the Island also has a special Vending Machine ~ Ocean Depths, where you can mainly vend clothing items. :)