[Babul]

do you mean the map boundaries from -3000,-3000 to 3000,3000 ? or do you want to detect the distance to the closest beach on a river/sea?
its possible to detect a player on land by checking the animation/height (and save that position on a timer/update/command/keypress), and restore the players' position if hes too far away while hes swimming/on z-ccordinates close to 0.0