Minecraft wiki entry for Endless_circling_pool, associated text: The sound that the water makes may disturb the player, so if you mind the noise, turn the computer's volume off.
Sometimes you may stop moving when in an AFK pool, and must manually move the player to start again. Using a second sign as the central block instead of a solid one can prevent this, and makes the pool more reliable.
Using an endless circling pool may be against the rules of your server, particularly if it has an auto rank system to give benefits for playing longer, or is often near the player cap. So, make sure to check the rules before constructing one.
Using this pool causes your hunger bar to go down, which can kill you in hard difficulty. Riding a boat in a pool wide enough (see #Endless circling moat below) does not deplete hunger. Otherwise, you keep food on hand for when you return, or make an automatic food farm and keep the use key pressed.
The pool should be encased in some sort of building, because otherwise you could be attacked by monsters and die.
Keep in mind that, if you are in a multiplayer server, other players might troll you when you are AFK.