Sometimes a tree really helps. I have bounced off a tree on the side right back into the middle of the fairway.
On the other hand, your rarely get a good bounce off the water.
On most courses, it is generally a 1 shot penalty. If you hit the first ball in, you can pull it out or drop down a new one, and access yourself 1 stroke. You are then hitting 3 from the point where you dropped it.
Most public courses also have a drop area where you can drop a new ball, and hit your 3rd shot. This drop area generally is not in line with the water.
As for a mercy rule, we always play one at "double par", so a 6 on a par 3.
On the rare occasions Ive golfed I discovered I’m incredibly proficient at putting the ball in the water. That’s a two stroke penalty right? Time to invoke the mercy rule around the time I hit 10 strokes on a par 3. Paul