There are a couple of decisions that shed some light on where boundaries should be:
33-2a/4 Where to Place Lines or Stakes Defining Margin of Water Hazard
Lines and stakes defining the margins of a water hazard should be placed as nearly as possible along the natural limits of the hazard, i.e., where the ground breaks down to form the depression containing the water. See also Decision 26-1/19.
26/3 Unmarked Water Hazard
Q. An unmarked ditch on the left of a hole is in bounds, but the left-hand margin is out of bounds. Accordingly, it is impossible to drop behind the water hazard under Rule 26-1b. A player’s ball comes to rest in the ditch. Is the player restricted to playing the ball as it lies or proceeding under Rule 26-1a?
A. It is the responsibility of the Committee to define accurately the margins of water hazards and lateral water hazards — see Rule 33-2a. However, if the Committee has not done so, the ditch is, by definition, a lateral water hazard and the player should be permitted to proceed under Rule 26-1c(i).