They are Old
There are many of them, countless hundreds. Any that are known are kept guarded. Despite their ubiquity, the art of making them and linking them has been lost. Usually found in small stone structures akin to mausoleums, they are deep circular pools of icy water. Many have been forgotten, many more have decayed, the thick stone that surrounds them not enough to stand against time.
All is not lost
But many are not lost, and the method to use them is still known. First, you must submerge yourself in the water. Completely. It is not recommended for the weak or aging as the icy water can easily sap the warmth from your body entirely. Then, a covering is laid across the pools entrance. This is anything from a metal grate to a stone slab. There must always be a gap between the covering and the water, however. This is for when you come out the other side.
Then, you hold your breath. You have to hold your breath up to the last possible moment. If you time it correctly, as you take your final desperate gasp for air you will appear at the exit pool. Well equipped drowning pools will have someone waiting at the other end with warm blankets and a fire. Less reputable pools leave you to take your own chances, though you still have to pay the fee of course.
If you go up for air too soon, the pool rips your soul from your body. This happens because the soul is transported much faster than the physical self, so as you push up for air you ground your body in the entry pool while your soul has already passed fully into the exit pool. This results in immediate death. A soul cannot survive for very long without a body, but given the right circumstances they will become ghosts. Many poorly maintained exit pools are haunted. Any professionals will keep a trained exorcist on the pay roll.
The Emperor Feyus, a confident swimmer, made regular use of drowning pools to travel between the bedrooms of his twin palaces that ruled the East and West of Dran. When Empress Direnz in West Dran discovered he also had an Empress in East Dran, she boarded up the exit pool on her side. It is generally considered unwise to use a Drowning Pool regularly, especially as a public figure.
Shady characters have also used Drowning Pools as a simple, more deadly alternative to highway robbery. After all, if your victim ends up drowned halfway across the continent it will be a long time before your reputation gets back to you.