/turf/simulated/water/update_icon() clears underlays before adding the new one

code/game/turfs/simulated/water.dm

@@ -19,6 +19,7 @@
 	..() // To get the edges.
 	icon_state = water_state
 	var/image/floorbed_sprite = image(icon = 'icons/turf/outdoors.dmi', icon_state = under_state)
+	underlays.Cut() // To clear the old underlay, so the list doesn't expand infinitely
 	underlays.Add(floorbed_sprite)
 	update_icon_edge()