Sean,
water vs sand - why is relief different. Most times, if the ball is in a water hazard, the ball cannot be found, or likely played. It can be found and likely played in a water hazard. If relief in the area of the point of entry into the water hazard, relief of one club-length would create disparity between right-handed and left-handed golfers. Depending on the situation, one or the other would likely have to stand in the hazard to continue. Making the relief two club-lengths eliminates most of the disparity. Above, I am talking about relief from a red marked hazard.
Further, unplayable lies are allowable in bunkers and through the green but not in water hazards. I believe the difference is that water hazards are sometimes dry. Since the penalty scenarios presume you are in the hazard, being able to take an unplayable lie
90% of the way across the water hazard, when it is dry in summer, or when the tide is lower, again creates a disparity.
Regarding sprinkler heads, etc. the rules have a continuity from the beginnings of golf, when there were virtually no man-made objects on the course. As these were added and when golfers encountered them disputes inevitably arose as one side of the match had a different opinion than the other side.
Many people think that reading Richard Tuft's "The Principles Behind the Rules of Golf" creates a better understanding of the issues. I have a spare copy if you are interested.