I'm sorry for asking such frequent questions, but I really don't understand. I searched the entire tower and all the shops, but I didn't find anything like it

A SFW turned based rpg with a heavy focus on transformation. · By
I don't mind, feel free to ask!
Small spoiler ahead!
You get the item to dive underwater from the warehouse in the south-western plains.
To get the key you need to have the Mermaid Form unlocked who you get in the tower.

Once you have the mermaid form you need to talk to this mermaid that gives you a new quest and area to explore. In the new area you can buy the Warehouse Key.
As for the other warehouse in town. You need to have a reputation of at least 400 to enter it.
I hope this helps. If not feel free to ask more!
I'm sorry for such a difficult question, but I have some problems with the shards of a fragment of the world. I found all the fragments, but there are only two shards. One from the castle... I don't remember where the second one was. I've been looking for a long time, but I can't find them.
I also have questions about kitsune form. Even after lighting all the torches, I got a dark one. What I did wrong? I got 26 forms. Do forms like Ryu and Phoenix exist ingame? Is there more secret forms than 3?
In the forest, near the point where you start as a fox, is a secret shop, it sells information of all shard locations with pictures, if you havent visited that place it might be a good start.
It means that you probaly have a too low reputation, simply doing a few more guild quest or other stuff that gives reputation should change that.
Yes they do exist, they are two out of the three secret tf's, they are ultra hard to find. If you dont want any spoiler on them, beating the game and unlocking ng+ gives hints on how and where to find them but they also can be found in the first playthrough, its just a lot harder.