Proper place to drop the ball?

Sorry, I'm on a roll here with all these posts because of several confusing situations I ran into over the weekend's round.

My buddy hit an approach shot over a pond onto the green, the ball went across the water, landed on the fringe of the green but then it rolled back down into the water.

One guy said that the rule is that he has to drop the ball "where it first entered the body of water" which is on this side of the pond and hit across again. I've always thought that the ball has to be dropped where it actually enters the water which is on the other side.

Can someone confirm where is the proper place to drop this ball?

First, you always use the point where the ball LAST crossed the margin of the hazard.  As far as the drop, it sounds like this was a regular water hazard with yellow stakes.  (Not lateral with red stakes.)  That being the case you keep the point where the ball last crossed the margin of the hazard between you and the pin, and drop the ball on an extension of that line away from the hole.

The diagram below gives you an idea of the drop.  If the ball crossed the other side of the pond, (greenside) and rolled back into the water. That would be the point where it last crossed.  You would have to keep that point between you and the pin and drop on a line or extension of that line.  This would put you back on your side of the pond.

6-1/1.5

Meaning of "Behind" in Rule 26-1

Q. With regard to the diagram, a player makes a stroke from the tee and his ball comes to rest in the water hazard at Point A, having last crossed the margin of the hazard at Point B. The player wishes to proceed under Rule 26-1b , which requires that the player drop a ball behind the water hazard, keeping the point at which the original ball last crossed the margin of the water hazard directly between the hole and the spot on which the ball is dropped. May the player drop a ball on dotted line Y-Y?

A. Yes. The reference to "behind" in Rule 26-1b means that the ball must be dropped outside the hazard behind the point where the ball last crossed the margin of the water hazard. Therefore, the player may drop a ball on either dotted line X-X or dotted line Y-Y.

26-1/2

Drop a ball behind the water hazard , keeping the point at which the original ball last crossed the margin of the water hazard directly between the hole and the spot on which the ball is dropped, with no limit to how far behind the water hazard the ball may be dropped; or

As additional options available only if the ball last crossed the margin of a lateral water hazard , drop a ball outside the water hazard within two club-lengths of and not nearer the hole than (i) the point where the original ball last crossed the margin of the water hazard or (ii) a point on the opposite margin of the water hazard equidistant from the hole .