Hi, firstly let say that you need no money to play castle Story. Gems are a useful commodity to enhance game play but there are loads of ways to earn gems now so don't worry about that.
Just let the goal take as long as it takes and it will take a few months. Drops of storm water are really rare but keep collecting from the mermaids anyway in the hope you get eels. Have you read through the notes and goal tips in the main goal guide or discussion thread? If not then it's well worth taking time to do so. Here's the thread you need for more info: https://forums.storm8.com/showthread...s-in-post-2-6*
Good luck! You'll get there