Why is it that whenever I make scrambled eggs they end up in a puddle of yellowish water?
As is so often the case in cooking, the culprit is overheated protein.
When you scramble eggs, initially the protein molecules in the eggs unfold into long strings that cross-bind with each other to form a network. That network holds the liquid and fat in suspension. The result is essentially a custard. If you overheat the eggs, though, the proteins start to contract, pulling in on themselves and tightening the network. Like wringing a towel, the water that is trapped gets squeezed out, and puddles on the bottom of the plate. The firmer you like your scrambled eggs cooked, the more likely this is to happen.
One cause of this is carry-over cooking - i.e. the eggs continue to cook a bit more even after you take them off of the heat. To avoid this, plate the eggs, preferably to a warmed (not hot) dish, when they are a little under-cooked to your liking. If you are going to leave them in the pan for a while before serving, take them off heat even sooner, as the residual heat of the pan will add to the carry-over cooking effect. Also, if at all possible, serve the the eggs immediately after they have been cooked. Holding them in a warm oven will only contribute to the amount of carry-over cooking they get, and could help to cause the watery puddle.
As an extra note, if you have ever been to France, you may have encountered eggs that are under cooked by North American standards. We generally tend to like our eggs cooked to a firm curd, whereas, at least in my experience, the French method is to stop at a softer curd surrounded by a creamy custard. The eggs are properly cooked and safe to eat this way, but it tends to put off the firm curd crowd.