Tracey and Cobweb10 months, 3 weeks ago
The link tells you what to plant for the fish - so if you wanted to attract Squirrelfish, for instance, you'd use Taiyo bonsai or red flowers or for Zebrafish you'd use the Tulipa Praecocias in the black or white magical pots, etc. Just follow the plants it says you need.
To get underwater you'd need to plant an underwater toadstool in any Fairytale, mushroom or Lottery pot. Even if you haven't done any Fairytales yet, you can still buy the pot for the first one and use that.
The seed will be in the Misc section of the seed shop, if you have earned any (you get them by spotting underwater wildlife).
One the toadstool is fully grown it attracts the mermaid, and when she's spotted she takes you underwater.