The well at Purapril Castle is connected to the sea by an underground waterway.
That makes a bit of sense I guess, but I still find the geography really weird.
The NPCs in Purapril village all make it obvious that the sea is my next destination. Also that desert area east of the tower makes my health go down so I can't go there yet.
Holy cow this ocean area is huge... I'm just exploring the surface for now.. found a house on an island where some green haired woman tells me about the shrine at the bottom of the sea that you need the amulet to get into, and asks me to trade my ocarina for info on how to get the amulet... says there's a door hidden between 4 trees.
Despite how huge this ocean area is, I found both the amulet and the shrine very quickly, both at the right-most side of the area. The shrine itself is just a very easy dungeon with lots of zora-like enemies and wind currents that you have to turn off by finding a switch.. there's also an above water part where you can fight these annoying jumping enemies and find some hearts and money... I found all of the treasures in this shrine, including the Oasis Boots which are needed to survive the desert.
Then I went back to the ocean and explored around there and got a bunch more treasures... the ocean itself is more of a maze than the shrine area. I also explored that water area near the beginning of the game, which is a different part of the ocean. I think I found all of the treasures I could for now, but there were some treasures I couldn't get to. I think there are hidden doors in here somewhere but I have no idea...
also, now the 2nd weapon shop in Purapril village is open and I'm able to get the steel shield and the excalibur.