I had a similar thought, but the idea was basically to drain the ocean into Narnia.

Magical gateways would usually have a torrent of air flowing through them due to varying pressure on either end.

If it's a "Once upon a time these worlds were the same place" or a "There used to be milions of these doors a billion years ago" kind of thing the pressure might be equal.

For the uninitiated: https://youtube.com/watch?v=AEtbFm_CjE0