Vincent, Im not exactly sure how the pros do it but I know there are alot of ways to get a decent accurate answer.
If you have a range-finder you could draw an overhead sketch of your pond. Then every twenty feet measure and record the distance across the pond...record the distance, standing point and the point the you measured to on your rough sketch. Then connect them by a line so you dont have a bunch of confusing points. Doing this you can get a real good estimate of your pond size. To get the area you have to do a little math. Too much detail for this post, but if you do have a rangefinder and want to do this just let me know when you get your data and I can calculate it.
Another option if you dont have a rangefinder but have a boat, is to do almost the same thing as above, but instead you use your boat, a timer, and then a speed gauge to calculate distance; recording as you go along. For instance, if you were going 10mph and it took 5 minutes to get across...then it would be roughly 880ft across. That's just an example. However, this method takes alot longer and is far less accurate.
Your other option is to hire a civil engineer and get him to survey the place. Not sure how much they cost per hour....but they could probably get it done fairly quickly
Chris
